[lldbtest] Fix self.filecheck check file lookup

The 'test_filename' property in TestBase changes over time, so
attempting to find a check file relative to the directory containing
'test_filename' is flaky.

Use the absolute path of the check file as that's always correct (and
simpler). This relies on the test driver changing into the test
directory, which it seems we can safely assume.

As a drive-by, make self.filecheck respect the trace (-t) option.

llvm-svn: 342699