### What does this PR try to resolve?
- separates each test into different test cases
- `snapbox` is used to test the snapshots
- difference in `.json` file alone should never cause a test to fail
- `.json` files updated only if expected fix != actual fix &&
`SNAPSHOTS=overwrite`
- when `.json` files are updated, the json is pretty printed
- replaced environment variables `RUSTFIX_TEST_*` for overwriting test
snapshots with `SNAPSHOTS=overwrite`
- ❗ The `RUSTFIX_TEST_RECORD_FIXED_RUST` feature is removed (generate a
`*.fixed.rs` on demand`). We can add it back whenever needed.
Fixes#13891
### How should we test and review this PR?
Run tests with:
```sh
cargo test -p rustfix
```
All the test should run as different test cases
nightly tests run only when using nightly version of rustc is used
### What does this PR try to resolve?
This is a horrible hack,
which lets the rustc invocation for learning target info always emit
warnings as warnings.
But at least it unblocks people who pass `-Awarnings` via RUSTFLAGS.
A long-term solution is a better interface
between build systems and the compiler.
See the discussion on Zulip:
https://rust-lang.zulipchat.com/#narrow/channel/131828-t-compiler/topic/Improving.20communication.20interface.20between.20cargo.2FrustcFixesrust-lang/cargo#8010
### How should we test and review this PR?
Ensure `CFG_DISABLE_CROSS_TESTS` is not set,
and run `cargo t --test testsuite
always_emit_warnings_as_warnings_when_learning_target_info`
This also additionally adds `wasm32-unknown-unknown` target to Cargo's
CI.
### Additional information
Instead of allowing any type in metadata, we were specifying fields like
`"string": "<any string>"`.
Found this when looking into #15030
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