Pull the tests from `types.rs` into `infer.rs`.
All of these are integration tests with the same basic form: create a
code sample, run type inference or check on it, and make some assertions
about types and/or diagnostics. These are the sort of tests we will want
to move into a test framework with a low-boilerplate custom textual
format. In the meantime, having them together (and more importantly,
their helper utilities together) means that it's easy to keep tests for
related language features together (iterable tests with other iterable
tests, callable tests with other callable tests), without an artificial
split based on tests which test diagnostics vs tests which test
inference. And it allows a single test to more easily test both
diagnostics and inference. (Ultimately in the test framework, they will
likely all test diagnostics, just in some cases the diagnostics will
come from `reveal_type()`.)