test-nointerrupt: make "sure" the handler "might" trigger (issue6558)

We are sure that the signal got sent in the right time frame, however, we still
have race, so either the code is actually buggy or we need some security to make
sure the signal get processed.

We might be affected by https://bugs.python.org/issue43406 ?

Differential Revision: https://phab.mercurial-scm.org/D11251

tests/test-nointerrupt.t

@@ -27,6 +27,8 b' Dummy extension simulating unsafe long r'
   >     with ui.uninterruptible():
   >         testing.write_file(sync_file, b'%d' % os.getpid())
   >         testing.wait_file(done_file)
+  >         # make sure we get rescheduled and the signal get a chance to be handled
+  >         time.sleep(0.1)
   >         ui.warn(b"end of unsafe operation\n")
   >     ui.warn(b"%d second(s) passed\n" % int(time.time() - start))
   > EOF
@@ -40,6 +42,7 b' Dummy extension simulating unsafe long r'
   > fi
   > "$RUNTESTDIR/testlib/wait-on-file" 10 "$SYNC_FILE" || exit 2
   > kill -s \$SIG \`cat "$SYNC_FILE"\`
+  > sleep 1
   > touch "$DONE_FILE"
   > EOF