tests/tcg: enable plugin testing

tests/tcg: enable plugin testing

If CONFIG_PLUGINS is enabled then lets enable testing for all our TCG
targets. This is a simple smoke test that ensure we don't crash or
otherwise barf out by running each plugin against each test.

There is a minor knock on effect for additional runners which need
specialised QEMU_OPTS which will also need to declare a plugin version
of the runner. If this gets onerous we might need to add another
helper.

Checking the results of the plugins is left for a later exercise.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>