Don't assume test case for %time will finish in 0 time.

Closes gh-2899

IPython/core/tests/test_magic.py

@@ -283,13 +283,13 b' def test_tb_syntaxerror():'
 def test_time():
     ip = get_ipython()
     
-    with tt.AssertPrints("CPU times: user 0 ns, sys: 0 ns, total: 0 ns"):
+    with tt.AssertPrints("CPU times: user "):
         ip.run_cell("%time None")
     
     ip.run_cell("def f(kmjy):\n"
                 "    %time print (2*kmjy)")
     
-    with tt.AssertPrints("CPU times: user 0 ns, sys: 0 ns, total: 0 ns"):
+    with tt.AssertPrints("CPU times: user "):
         with tt.AssertPrints("hihi", suppress=False):
             ip.run_cell("f('hi')")