Enable some previously omitted test scripts. Add a test cross-check.

Enable some previously omitted test scripts.  Add a test cross-check.

* tests/rm/Makefile.am (TESTS): Add empty-name and unreadable.
* tests/rm/empty-name: Likewise.
* tests/rm/unreadable: Reenable this test.  Adjust for new rm.
* tests/mkdir/Makefile.am: Add writable-under-readonly.
* tests/mkdir/writable-under-readonly: Add some comments.
This test is always skipped, for now.
* tests/Makefile.am (ta): Hook up the new root-only script.
* tests/tail-2/infloop-1: Make this test pass.
* tests/tail-2/Makefile.am (TESTS): Add infloop-1.
* tests/tail-2/fflush: Remove unused file.
* tests/check.mk (vc_executable_is_in_TESTS): More portable.
* tests/check.mk (check): Depend on the above.
* build-aux/check.mk: Remove comment mentioning AUTHORS file.