tests: Run "timeout tests" sequentially

Merged Bastien Nocera requested to merge wip/hadess/socket-test-not-parallel into master

Those tests seem to regularly fail because a timeout (which we're measuring outside the function that times out) is too long, which can happen when the system is busy.

Try not running those tests in parallel to somewhat lower the incidence of failures.

Bail out! GLib-GIO:ERROR:../gio/tests/socket.c:1167:test_timed_wait: assertion failed (poll_duration < 112000): (114254 < 112000)
Bail out! GLib-GIO:ERROR:../gio/tests/cancellable.c:167:on_mock_operation_ready: assertion failed (error == (g-io-error-quark, 19)): error is NULL
Edited by Bastien Nocera

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