tests: avoid running duplicate qom-tests

Since 3687d532 we've been unconditionally adding qom-test to our qtests
for every arch. However, some archs inherit their tests from Makefile
variables for other archs, such as i386/x86_64,
microblaze/microblazeel, and xtensa/xtensaeb. Since these are evaluated
in a lazy manner, we ultimately end up adding qom-test twice.

In the case x86_64, where we have a large number of machine types that
we rerun qom-test for, this has lead to a fairly noticeable increase
in the overall run-time of `make check` (78s vs. 42s on my machine).
Similar speed-ups are visible for other such archs, but not nearly as
significant.

Fix this by only adding qom-test to an arch's test list if it's not
already present.

Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Andreas Färber <afaerber@suse.de>
Cc: qemu-stable@nongnu.org
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
