fix: stop tests from deleting the system temp dir and FileUtils.delete from following symlinks

[AhmedYoussef98]

Purpose of the pull request

Closed: #1169

What's changed?

• CompatibilityTest#readWithCacheAfterTempDeletion now deletes only Fesod's own temp directory (FileUtils.getTempFilePrefix()) and no longer deletes the whole java.io.tmpdir. That is still what the test means to check: Fesod recovers when its temp and cache files are removed while idle. The system temp directory is shared with other processes, so a test must never delete it.
• FileUtils.delete now deletes a symbolic link itself and does not follow it. Before, File.isDirectory()/listFiles() resolved links, so the recursive delete could leave the directory it was given. On macOS, an Applications -> /Applications link inside a mounted installer DMG under $TMPDIR led it to wipe application bundles (details in [Bug] CompatibilityTest deletes the whole system temp dir, and FileUtils.delete follows symlinks out of it #1169).
• Adds FileUtilsTest#test_delete_doesNotFollowSymbolicLinks. It fails on current main (the link target's contents are deleted) and passes with this change. It is skipped where symbolic links can't be created.

Verified with ./mvnw package -Dmaven.test.skip=false -Dtest='FileUtilsTest,CompatibilityTest' -pl fesod-common,fesod-shaded,fesod-sheet (27 tests, all passing) and ./mvnw spotless:check.

Checklist

• I have read the Contributor Guide.
• I have written the necessary doc or comment.
• I have added the necessary unit tests and all cases have passed.