gh-130917: update workload for test_signal.test_itimer_virtual

The workload to advance the virtual timeout is too lightweight for
some platforms. As result the test goes in timeout as it never reaches
the end of the timer.
By having a heavier workload, the virtual timer advances rapidly and the
SIGVTALRM is sent before the timeout.

Lib/test/test_signal.py

@@ -842,7 +842,7 @@ def test_itimer_virtual(self):
 
         for _ in support.busy_retry(support.LONG_TIMEOUT):
             # use up some virtual time by doing real work
-            _ = pow(12345, 67890, 10000019)
+            _ = sum(i * i for i in range(10**5))
             if signal.getitimer(self.itimer) == (0.0, 0.0):
                 # sig_vtalrm handler stopped this itimer
                 break
@@ -859,7 +859,7 @@ def test_itimer_prof(self):
 
         for _ in support.busy_retry(support.LONG_TIMEOUT):
             # do some work
-            _ = pow(12345, 67890, 10000019)
+            _ = sum(i * i for i in range(10**5))
             if signal.getitimer(self.itimer) == (0.0, 0.0):
                 # sig_prof handler stopped this itimer
                 break