TST: Catch possible warnings

Previously the test sometimes fails with the following error:

======================================================================
ERROR: Test a special case for var
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/travis/virtualenv/python3.2/lib/python3.2/site-packages/numpy/ma/tests/test_core.py", line 2731, in test_varstd_specialcases
    _ = method(out=mout)
  File "/home/travis/virtualenv/python3.2/lib/python3.2/site-packages/numpy/ma/core.py", line 4778, in std
    np.power(out, 0.5, out=out, casting='unsafe')
RuntimeWarning: invalid value encountered in power

----------------------------------------------------------------------

Now we catch it, as recommended by the thread:

http://old.nabble.com/Should-abs(-nan-)-be-supported--td34389839.html

numpy/ma/tests/test_core.py

@@ -2728,11 +2728,23 @@ def test_varstd_specialcases(self):
             self.assertTrue(method(0) is masked)
             self.assertTrue(method(-1) is masked)
             # Using a masked array as explicit output
-            _ = method(out=mout)
+            warn_ctx = WarningManager()
+            warn_ctx.__enter__()
+            try:
+                warnings.simplefilter('ignore')
+                _ = method(out=mout)
+            finally:
+                warn_ctx.__exit__()
             self.assertTrue(mout is not masked)
             assert_equal(mout.mask, True)
             # Using a ndarray as explicit output
-            _ = method(out=nout)
+            warn_ctx = WarningManager()
+            warn_ctx.__enter__()
+            try:
+                warnings.simplefilter('ignore')
+                _ = method(out=nout)
+            finally:
+                warn_ctx.__exit__()
             self.assertTrue(np.isnan(nout))
         #
         x = array(arange(10), mask=True)