Follow-up from "GaudiToolbox: do not install test executables"

The following discussion from !1797 (merged) should be addressed:

• @clemenci started a discussion: (+1 comment)

  The reason why I didn't add TEST here is that it implies a

  add_test(${target})

  but the catch_discover_tests(test_AlgContextSvc ...) a few lines below invokes one add_test for each of the test cases in the generated executable, resulting in duplicate execution of tests.

  Something similar is possible with Google test too.

  We need proper support for tests harnesses that do not use the plain add_test.