tests/qtest: tighten up the checks on clock_step

It is invalid to call clock_step with an implied time to step forward
as if no timers are running we won't be able to advance.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20250207153112.3939799-7-alex.bennee@linaro.org>
1 file changed