tests: skip individual tests rather than whole test programs

Use g_skip_test() to skip individual tests rather than just returning
status 77 from the test program as a whole.

In several cases, we still end up skipping more than necessary, due to
test cases that need to be split up more.

Remove the "MISSING_REGRESSION_TESTS_PACKAGES" functionality, since
the skipped tests are now pointed out explicitly.
2.45.90