test-symlinkat: enhance test

test-symlinkat: enhance test

* tests/test-readlink.c (main): Move guts...
* tests/test-readlink.h (test_readlink): ...into new file.
* tests/test-symlink.c (main): Move guts...
* tests/test-symlink.h (test_symlink): ...into new file.
* tests/test-symlinkat.c (main): Use new files for further
coverage.
(do_symlink, do_readlink): New helper functions.
* modules/symlink-tests (Files): Ship new file.
(Depends-on): Add stdbool.
* modules/readlink-tests (Files): Ship new file.
(Depends-on): Add stdbool.
* modules/symlinkat-tests (Files): Use new files.

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