[Dexter] Add option to Dexter to name results based on directory (#148611)
As a legacy of Dexter's role as a test runner, it selects a name for
result files based on the relative path from the test root to each
individual test. Since Dexter no longer takes a test directory as an
argument, only the basename for each test is ever used. This patch adds
an optional --test-root-dir argument, allowing for relative paths to be
used for result files again.