Make run-clang-tidy.py print the configured checks correctly

The test invocation at the start of run-clang-tidy.py (line 257) prints
all enabled checks - meaning either the default set or anything
configured via the -checks option. If any checks were (un-)configured
via the -config option, these are not printed. This is confusing to the
user, since the list of checks that are printed may be different from
the list of checks that are used by the non-testing calls to clang-tidy,
where the -config option is passed correctly.

This patch adds the -config option to the test invocation of clang-tidy
at the start of the script. This means that checks (un-)configured via
the -config option (rather than the -checks option) are applied
correctly, when printing the list of enabled checks.