Bring harmony to the test snapshot filtering situation (#2678)

The snapshot filtering situation has gotten way out of hand, with each
test hand-rolling it's own filters on top of copied cruft from previous
tests.

I've attempted to address this holistically:

- `TestContext.filters()` has everything you should need 
- This was introduced a while ago, but needed a few more filters for it
to be generalized everywhere
- Using `INSTA_FILTERS` is **not recommended** unless you do not want
the context filters
    - It is okay to extend these filters for things unrelated to paths
- If you have to write a custom path filter, please highlight it in
review so we can address it in the common module
- `TestContext.site_packages()` gives cross-platform access to the
site-packages directory
    - Do not manually construct the path to site-packages from the venv
- Do not turn off tests on Windows because you manually constructed a
Unix path to site-packages
- `TestContext.workspace_root` gives access to uv's repository directory
    - Use this for installing from `scripts/packages/`
- If you need coverage for relative paths, copy the test package into
the `temp_dir` don't change the working directory of the test fixture

There is additional work that can be done here, such as:

- Auditing and removing additional uses of `INSTA_FILTERS`
- Updating manual construction of `Command` instances to use a utility
- The `venv` tests are particularly frightening in their lack of a test
context and could use some love
- Improving the developer experience i.e. apply context filters to
snapshots by default

scripts/scenarios/templates/compile.mustache

@@ -64,7 +64,7 @@ fn {{module_name}}() -> Result<()> {
     let python_versions = &[{{#environment.additional_python}}"{{.}}", {{/environment.additional_python}}];
 
     // In addition to the standard filters, swap out package names for shorter messages
-    let mut filters = INSTA_FILTERS.to_vec();
+    let mut filters = context.filters();
     filters.push((r"{{name}}-", "package-"));
 
     let requirements_in = context.temp_dir.child("requirements.in");

scripts/scenarios/templates/install.mustache

@@ -82,7 +82,7 @@ fn {{module_name}}() {
     let context = TestContext::new("{{environment.python}}");
 
     // In addition to the standard filters, swap out package names for shorter messages
-    let mut filters = INSTA_FILTERS.to_vec();
+    let mut filters = context.filters();
     filters.push((r"{{name}}-", "package-"));
 
     uv_snapshot!(filters, command(&context)