tests/tcg: allow to check output of plugins

tests/tcg: allow to check output of plugins

A specific plugin test can now read and check a plugin output, to ensure
it contains expected values.

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: <20240724194708.1843704-5-pierrick.bouvier@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20240916085400.1046925-8-alex.bennee@linaro.org>