Revise ext/standard/tests/file test suite

Description

Occasionally, there are still test conflicts, and a couple of tests are marked as obscure_filename conflicting. Furthermore, some of the tests do not necessarily clean up properly after execution (they unlink some created files during test execution only). Also, some tests appear to have POSIX and Windows variants, although that doesn't seem to be necessary. See e.g. #16722 and #16854.

Ideally, all tests should only create and delete files which either have the same base name as the test case, or are placed in a directory with the base name of the test case. Since this may lead to overlong paths, we may need to take some shortcuts there (possibly using shorter test case names).

In any way, this test suite should be cleaned up/improved.

Esamina prima ext/standard/tests/file e gli esempi correlati in #16722 e #16854. Fai un inventario dei conflitti, delle marcature obscure_filename, del comportamento di pulizia e delle varianti POSIX/Windows; il lavoro è completo quando la suite isola in modo coerente i propri file o le proprie directory e rimuove i file creati senza varianti non necessarie.