This more or less rewrites this test so that it explicitly tests the
"interpolation" when a timeline loops, i.e. that if something occupies
the thread when a timeline was supposed to have looped, we end up in the
right place "in the middle" of the next timeline cycle.
The test more or less does this:
* Start a 3 second looping timeline
* Sleep so that we're in the middle of the first cycle
* Sleep again so that we end up in the middle of the next cycle
The semantics checked are that we see the following frames:
* The first frame with timestamp 0
* The second frame in the middle of the first cycle (timestamp ~= 1.5
sceonds)
* The third frame in the end of the first cycle (timestamp == 3.0
seconds)
* The fourth frame, first in the second cycle, with timestamp ~= 1.5
seconds)
This means we can increase the "grace period" to the double (from 0.5 s
to 1 s), while at the same time decrease the time spent running the test
(from 10 s to 4.5 s). This should hopefully make the test less flaky,
especially in slower runners, e.g. aarch64.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1751>
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