docs/devel: document how to run individual TCG tests

Since 6f6ca067d2 (tests/tcg: add some help output for running
individual tests) we made it easier to run individual tests for a
given architecture. Lets reference that in the developer
documentation.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20240729144414.830369-8-alex.bennee@linaro.org>
1 file changed