bpo-46442: improve and rename testExceptionCleanupNames (GH-30758)

The test tested that explicitly deleting the local variable bound to the exception
did not cause problems, but it did not test what it actually claimed to test, i.e.
that the variable is deleted automatically.
(cherry picked from commit 82c53229e18f5853c82cb8ab6b9af1925a0e9e58)

Co-authored-by: Yellow Dusk <yellow.dusk1590@fastmail.com>