Hopefully fix test_cancel_from_signal flakiness on testnodes

After inspecting nxdtest status on testnodes I see frequent failures of
test_cancel_from_signal - from e.g. https://erp5js.nexedi.net/#/test_result_module/20250218-F232A924/2 :

    =================================== FAILURES ===================================
    _________________ test_cancel_from_signal[userns_default-sig0] _________________

        def _():
            proc.terminate()
            if proc.poll() is None:
    >           time.sleep(1)
    E           Failed: Timeout >3.0s

    nxdtest/nxdtest_test.py:385: Failed

which means, as that _() is executed upon exiting from test_cancel_from_signal
function, that time.sleep(1) simply timed out exceeding 3 seconds of total
time budget we gave to this test to run.

Now if I look at b0cf277d (Cancel test run on SIGINT/SIGTERM), that
introduced this test, I can see that I even put another
pytest.mark.timeout(timeout=10) there in commented form, and probably
forgot to switch back from pytest.mark.timeout(timeout=3) I used during
local debugging.

The other tests, that use timeouts, all use 10s of time budget out of the box:

    test_run_procleak        0ad45a9c (Detect if a test leaks processes and terminate them)
    test_cancel_from_master  5d656ccf (Add test for cancel propagation)

so, given that OS load is high on testnodes machines, I think we should
do the same for test_cancel_from_signal as well.

This should hopefully fix the run-out timeouts for this test we
currently see on testnodes.

/reviewed-by @jerome
/reviewed-on !20

nxdtest/nxdtest_test.py

 # -*- coding: utf-8 -*-
-# Copyright (C) 2020-2022  Nexedi SA and Contributors.
+# Copyright (C) 2020-2025  Nexedi SA and Contributors.
 #
 # This program is free software: you can Use, Study, Modify and Redistribute
 # it under the terms of the GNU General Public License version 3, or (at your
@@ -365,8 +365,7 @@ TestCase('TEST1', ['%s'])
 
 
 # verify that nxdtest cancels test run on SIGINT/SIGTERM.
-#@pytest.mark.timeout(timeout=10)
-@pytest.mark.timeout(timeout=3)
+@pytest.mark.timeout(timeout=10)
 @pytest.mark.parametrize('sig', [(signal.SIGINT, "Interrupt"), (signal.SIGTERM, "Terminate")])
 @func
 def test_cancel_from_signal(tmpdir, sig):