generic: test for creating duplicate filenames in encrypted dir

generic: test for creating duplicate filenames in encrypted dir

Test for a race condition where a duplicate filename could be created in
an encrypted directory while the directory's encryption key was being
added concurrently.

generic/595 was already failing on ubifs due to this bug, but only by
accident.  This new test detects the bug on both ext4 and ubifs.  I
wasn't able to get it to detect the bug on f2fs.

Signed-off-by: Eric Biggers <ebiggers@google.com>