ctdb-tests: Fix logic to handle PATH additions for tests

ctdb-tests: Fix logic to handle PATH additions for tests

When using non-standard test subdirectories, the current code can fail
to find the test bin directory and stupidly just adds /bin to PATH.

Switch to using CTDB_TESTS_ARE_INSTALLED along with some sanity checks
to determine the mode of operation.

With this change, test directories can now be created as
subdirectories of arbitrary component directories.  Tests can then be
run directly, either by specifying the subdirectory or individual test
cases.

Integration into the top-level tests/ directory is then done via a
symbolic link, which enables 2 things:

* Ability to run a directory of test cases from top level by simply
  specifying the link name.

* Ease of installation - the installation code just works with the
  symbolic link.

Signed-off-by: Martin Schwenke <martin@meltin.net>
Reviewed-by: Amitay Isaacs <amitay@gmail.com>