CI: Remove pass-through test-functionality test wrappers (#1383)

CI: Remove pass-through test-functionality test wrappers (#1383)

Instead of requiring a custom test wrapper for each test and, hence,
creating an ever-increasing number of pass-through wrappers that do
nothing useful, use a custom test wrapper if and only if it exists. By
default (i.e. when there is no custom wrapper), just run the named test.

As a positive side effect, this change also simplifies running tests
that are not on the $default_tests list hard-coded in main():

    test-suite/test-functionality.sh some-new-test-i-am-working-on