tests/tcg: limit the scope of the plugin tests

tests/tcg: limit the scope of the plugin tests

Running every plugin with every test is getting excessive as well as
not really improving coverage that much. Restrict the plugin tests to
just the MULTIARCH_TESTS which are shared between most architecture
for both system and user-mode. For those that aren't we need to squash
MULTIARCH_TESTS so we don't add them when they are not part of the
TESTS global.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Acked-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20230424092249.58552-14-alex.bennee@linaro.org>