oeqa: reproducible: Fix test not producing diffs

Diffoscope changed the --exclude-directory-metadata option to require an
argument.

Add a test to validate that diffoscope is functioning as
expected to ensure that future upgrades do not unintentionally break
the reproducibility tests.

[YOCTO #14025]

Signed-off-by: Joshua Watt <JPEWhacker@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit ea8fbcb7)
Signed-off-by: Steve Sakoman <steve@sakoman.com>