tests-clock: Fix race in test_late_crank
There was a case where we started waiting on the clock before setting the clock time, leading to the wait succeeding instead of being late:
gsttestclock.c:1073:F:testclock:test_late_crank:0: '1 * GST_SECOND' (1000000000) is not equal to 'context.jitter' (-4000000000)
Fixes #426 (closed)
Co-authored by: Mathieu Duponchelle mathieu@centricular.com