tests/tcg: move test plugins into tcg subdir

tests/tcg: move test plugins into tcg subdir

You cannot use plugins without TCG enabled so it doesn't make sense to
have them separated off in the test directory structure. While we are
at it rename the directory to plugins to reflect the plural nature of
the directory and match up with contrib/plugins.

Suggested-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20240729144414.830369-10-alex.bennee@linaro.org>