Avoid installcheck failure in TAP tests using injection_points

Avoid installcheck failure in TAP tests using injection_points

These tests depend on the test module injection_points to be installed,
but it may not be available as the contents of src/test/modules/ are not
installed by default.

This commit adds a workaround based on a scan of pg_available_extensions
to check if the extension is available, skipping the test if it is not.
This allows installcheck to work transparently.

There are more tests impacted by this problem on HEAD, but for now this
addresses only the tests that exist on HEAD and v17 as the release is
close by.

Reported-by: Maxim Orlov
Discussion: https://postgr.es/m/CACG=ezZkoT-pFz6a9XnyToiuR-Wg8fGELqHLoyBodr+2h-77qA@mail.gmail.com
Backpatch-through: 17