tests: root_test: use TMPDIR=/var/tmp

Otherwise we fail like this on my Fedora 38 box:

	=== RUN   TestOverlay
	DetectQuirks: tmpfs detected, no extended attributes except acls will work.
	    root_test.go:379: No user xattrs! overlay mount will likely fail.
	15:15:57.957960 Unimplemented opcode OPCODE-51
	    root_test.go:398: mount: /tmp/gocryptfs-test-parent-0/3652394902/TestOverlay.2374697046.mnt/merged: wrong fs type, bad option, bad superblock on overlay, missing codepage or helper program, or other error.
	               dmesg(1) may have more information after failed mount system call.

	    root_test.go:399: exit status 32
	--- FAIL: TestOverlay (0.04s)
	FAIL

Also fix the messed-up DetectQuirks bit test.

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