Tests: Skip tests in test_*.sh if encoders and/or decoders are disabled.

Tests: Skip tests in test_*.sh if encoders and/or decoders are disabled.

This isn't perfect as the scripts can still fail if only
certain filters are disabled. This is still an improvement
as now "make check" has better behavior when all encoders
or decoders are disabled.

Grepping ../config.h is simple and fairly clean but it only
works if config.h was created. CMake builds don't create
config.h but they don't use these test scripts either.

Thanks to Sebastian Andrzej Siewior for reporting the problem.
Thanks to Jia Tan for the original patch which grepped xz
error messages instead of config.h.