Rollup merge of #123568 - Oneirical:delete-tests, r=wesleywiser

Clean up tests/ui by removing `does-nothing.rs`

In [a previous PR](#123297 (comment)), it was suggested that this test be removed:

> it's testing a basic diagnostic for an unknown variable (added over a decade ago for #154) that is already covered by probably dozens or hundreds of other tests.

It was then suggested that [opening a new PR](#123563 (comment)) for this would be more organized.

I'm setting this as a draft, as:

1. The tests/ui directory is rather disorganized, a large quantity of tests are not even contained inside their own directories. This PR could turn into "clean up the UI tests directory", if I were to place everything into categories (for example, everything related to CLI flags could get placed in a cli directory).
2. This will have a merge conflict with #123563 should that get merged. I trust that _this time_, I won't run into [The Incident](#123297 (comment)) while rebasing. Edit: Yay, I did it properly!

Author: matthiaskrgr

src/tools/tidy/src/ui_tests.rs

@@ -18,7 +18,7 @@ const ENTRY_LIMIT: usize = 900;
 // FIXME: The following limits should be reduced eventually.
 
 const ISSUES_ENTRY_LIMIT: usize = 1722;
-const ROOT_ENTRY_LIMIT: usize = 861;
+const ROOT_ENTRY_LIMIT: usize = 859;
 
 const EXPECTED_TEST_FILE_EXTENSIONS: &[&str] = &[
     "rs",     // test source files