docs: Document TIMEOUT_MULTIPLIER for raising test timeouts

Our test infrastructure allows you to set the TIMEOUT_MULTIPLIER
environment variable to raise the test timeouts if you're building
for a slow environment.  (scripts/mtest2make.py reads it and sets the
meson test -t argument accordingly.)

Document this so it's not a secret feature only known to a select
few.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Pierrick Bouvier <pierrick.bouvier@oss.qualcomm.com>
Message-id: 20260427161132.1463385-1-peter.maydell@linaro.org