tests: tighten link, rmdir, and remove tests

tests: tighten link, rmdir, and remove tests

Catch cygwin bug where rmdir("a/./") failed but rmdir("a/.//")
succeeded.  Be kinder if a Solaris root user runs the test.

* tests/test-link.h (includes): No need to use <config.h> here.
Clean up if directory hard link was created, otherwise test for
trailing '.'.
* tests/test-linkat.c (main): Simplify.
* tests/test-remove.c (main): Enhance test for trailing '.'.
* tests/test-rmdir.h (test_rmdir_func): Likewise.

Signed-off-by: Eric Blake <ebb9@byu.net>