tests/tcg: add mechanism to run specific tests with plugins

tests/tcg: add mechanism to run specific tests with plugins

Only multiarch tests are run with plugins, and we want to be able to run
per-arch test with plugins too.

Tested-by: Xingtao Yao <yaoxt.fnst@fujitsu.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Message-Id: <20240724194708.1843704-4-pierrick.bouvier@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20240916085400.1046925-7-alex.bennee@linaro.org>