test: fix flaky test_cancel_from_master

We did not wait long enough for the tested process to set their signal
handler, as we can see on test history [1], this test fails often when
running on test node.

It always pass when I run locally, but if I adjust the _tmasterpoll
patch set to 0.01 second, it fails the same way.

1: https://erp5.nexedi.net/test_result_module/20220616-13A460D9B/2/TestResultLine_viewResultHistory

/reviewed-on nexedi/nxdtest!17