[6.x] Only recreate the fake Stache directory when it's gone

[joshdaugherty]

Fixes #15537

deleteFakeStacheDirectory() called mkdir() straight after deleteDirectory(). deleteDirectory() ignores a failed rmdir(), and on Windows a directory stays in place while any process still has a handle open on it. So the directory could still exist, and mkdir() threw ErrorException: mkdir(): File exists in tearDown(), failing a test that had passed.

It now recreates the directory only if it's gone. The directory that survives is already empty, since only the final rmdir() failed, so the next test still starts from a clean fake Stache. This only changes test tooling (PreventsSavingStacheItemsToDisk, used by Tests\TestCase and AddonTestCase).

Tests

Adds tests/Testing/Concerns/PreventsSavingStacheItemsToDiskTest.php:

• it empties and recreates the fake stache directory, the normal case.
• it does not fail when the directory outlives its deletion, which binds files to a Filesystem whose deleteDirectory() keeps the directory, like Windows does while a handle is open. On 6.x without the change it throws mkdir(): File exists; with it, it passes.

tests/Stache and tests/Antlers/Parser/ComponentTagsTest.php, which use the trait, still pass (221 tests).