Modify test so that it looks for patterns in stderr as well

With the change https://reviews.llvm.org/D33013 driver will not build
compilation object if command line is invalid, in particular, if
unrecognized option is provided. In such cases it will prints diagnostics
on stderr. The test 'clang-tidy/diagnostic.cpp' checks reaction on
unrecognized option and will fail when D33013 is applied because it checks
only stdout for test patterns and expects the name of diagnostic category
prepared by clang-tidy. With this change the test makes more general check
and must work in either case.

Differential Revision: https://reviews.llvm.org/D33173

llvm-svn: 303735