Add a tidy check to prevent from adding a new test to `src/test`

[JohnTitor]

The diff is huge and I cannot check on web UI, but is there a tidy check that prevents contributors from adding a test to src/test?
A PR modifying an existing test now has a merge conflict e.g. #106167 but a PR adding a new test doesn't e.g. #106660, and I'm worried if a new test is added to src/test by accident.

Originally posted by @JohnTitor in #106458 (comment)

[jyn514]

Mentoring instructions: Add a new module to src/tools/tidy, then call the check from

rust/src/tools/tidy/src/main.rs

Line 90 in b22c152

// Checks that need to be done for both the compiler and std libraries.

. See the surrounding code for examples of how tidy checks work.

[BoxyUwU]

I am confused as to why this was not a blocker for moving the tests

[jyn514]

@BoxyUwU "I didn't think of it"

[albertlarsan68]

@rustbot claim