Use a larger runner for tests (#889)

Alternative to #875. Instead of partitioning tests across multiple
runners via nextest, we use a larger GitHub Actions runner.
Additionally, we explore using nextest to take advantage of the
increased number of cores.

On the 8-core machine, nextest is 22% faster than insta. In combination
with the vastly more readable output, I think this means we should
switch over. As noted in #875 we lose the ability to detect unreferenced
snapshot files but since we inline all of our snapshots this shouldn't
matter.

### Benchmarks

The following are the runtime of _just_ the test portion of the test job
in GitHub Actions except the partitioned case from #875 which requires a
separate build step making runner overhead relevant.

The compile times are noted as a reference as a possible lower bound of
test times. The compile time **is included** in all of the test times
shown.

Where the nextest thread count is not noted, it is inferred from the CPU
count.

```
test                                  time        diff
------------------------------------------------------
2-core (main)                         4m 53s
2-core-nextest-partioned (#875)       3m 56s      -19%
4-core-compilation                       32s      
4-core-insta                          1m 47s      -63%
4-core-nextest                        1m 40s      -66%
8-core-compilation                       18s      
8-core-insta                          1m  9s      -76%
8-core-nextest                        1m  5s      -78%
8-core-nextest-12-threads                54s      -82%
8-core-nextest-16-threads                55s      -82%
```


### Cost

We must pay per-minute costs for these runners:

> Larger runners are not eligible for the use of included minutes on
private repositories. For both private and public repositories, when
larger runners are in use, they will always be billed at the per-minute
rate.
>
> Compared to standard GitHub-hosted runners, larger runners are billed
differently. Larger runners are only billed at the per-minute rate for
the amount of time workflows are executed on them.


[[source]](https://docs.github.com/en/actions/using-github-hosted-runners/about-larger-runners/about-larger-runners#understanding-billing)

The per-minute rates are as follows:

> Linux	2	$0.008  (main)
> Linux	4	$0.016
> Linux	8	$0.032  (pull request)


[[source]](https://docs.github.com/en/billing/managing-billing-for-github-actions/about-billing-for-github-actions#per-minute-rates)

The per-minute cost increases by 4x but the workflow is 5.2x faster
since we are making use of the extra compute. We will not get any free
minutes executing these runners once the repository is public.
Additionally, we will not make use of our [3,000 minutes /
month](https://docs.github.com/en/billing/managing-billing-for-github-actions/about-billing-for-github-actions#included-storage-and-minutes)
of included minutes. Using the 8-core machines, the included 3,000
minutes should account for approximately ~$100.

Here's a brief analysis of costs from the last few
```

Minutes used
------------
November 1090 + 3000 = 4090
December 1357 + 3000 = 4357
January  2655 in 7 days
         ~3x more expected
                     = 11000 estimated

Costs
-----
November  1090 * 0.008   = $ 8.72
December  1357 * 0.008   = $10.86
January   8000 * 0.008   = $64 projected using 3000 included minutes and 2-core machines
          (11000 - (0.82 * 11000)) * 0.032
                         = $63 projected without included minutes and 4-core machines with perf improvement
          (11000 - (0.70 * 11000)) * 0.032
                         = $100 projected with a more conservative 70% reduction in total runtime

```

We can reduce costs (once public) by disabling larger runners for
non-organization users e.g.
https://github.com/PrefectHQ/prefect/pull/9519

.github/workflows/ci.yaml

@@ -46,7 +46,11 @@ jobs:
     strategy:
       matrix:
         os: [ubuntu-latest]
-    runs-on: ${{ matrix.os }}
+    runs-on:
+      # We use the large GitHub actions runners for faster testing
+      # For Ubuntu and Windows, this requires Organization-level configuration
+      # See: https://docs.github.com/en/actions/using-github-hosted-runners/about-larger-runners/about-larger-runners#about-ubuntu-and-windows-larger-runners
+      labels: ${{ matrix.os }}-large
     name: "cargo test | ${{ matrix.os }}"
     steps:
       - uses: actions/checkout@v4
@@ -67,9 +71,12 @@ jobs:
         uses: taiki-e/install-action@v2
         with:
           tool: cargo-insta
+      - name: "Install cargo nextest"
+        uses: taiki-e/install-action@v2
+        with:
+          tool: cargo-nextest
       - uses: Swatinem/rust-cache@v2
         with:
           save-if: ${{ github.ref == 'refs/heads/main' }}
-      - name: "Tests (Ubuntu)"
-        if: ${{ matrix.os == 'ubuntu-latest' }}
-        run: cargo insta test --all --all-features --unreferenced reject
+      - name: "Tests"
+        run: cargo nextest run --all --all-features --status-level skip --failure-output immediate-final --no-fail-fast -j 12