Allow more time on the concurrency test

Spawning processes is apparently really slow on Windows, and the test is
occasionally taking slightly more than a second on AppVeyor. I think we
can allow up to the full 2.5 seconds without losing the valuable bits of
the test.

tests/xargs_test.py

@@ -177,4 +177,6 @@ def test_xargs_concurrency():
     assert ret == 0
     pids = stdout.splitlines()
     assert len(pids) == 5
-    assert elapsed < 1
+    # It would take 0.5*5=2.5 seconds ot run all of these in serial, so if it
+    # takes less, they must have run concurrently.
+    assert elapsed < 2.5