tests/tcg: clean-up some comments after the de-tangling

tests/tcg: clean-up some comments after the de-tangling

These were missed in the recent de-tangling so have been updated to be
more actuate. I've also built up ARM_TESTS in a manner similar to
AARCH64_TESTS for better consistency.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>