custom-definesandincludes tests: drop trying to load own plugin, also per bad id

Update: now split into two commits per case.

test_compilerprovider does not use the plugin, but links the shared library to access the code to test instead.

test_definesandincludes: does not need to try to load the plugin, is always loaded anyway.

And in any case both thried to do this using a bad id anyway.