Skip language controller test data rows for missing optional plugins

All 4 language plugins - clang, CMake, diff, qml/js - are optional.
Either due to ecm_optional_add_subdirectory() or due to an optional
external dependency, i.e. KompareDiff2 for the patchreview plugin.

7f239a71 skipped patchreview's test data
rows if the plugin was missing. The qmljs plugin has not been ported to
Qt 6 and is currently unconditionally disabled, which makes its test
data rows fail. A general solution implemented by this commit fixes
current test failures and should prevent future similar ones.