Filter warnings in rcparams test.
Suppress some more warnings.

These are genuine too so we filter them
Catch warnings in tightlayout test.

These are warnings that are raised because tightlayout does not work
fully automatic on gridspec. However, the point of the test is doing
this with some manual help so this works as expected.
Add note about disabling warnings in test_rcparams
Add note to test_subplots
Add note to test_tightlayout
Suppress deprecation warning for Delaunay module in the test suite
Suppress warning due to NaNs in this test.
Suppress warning when handling NaNs in streamplot
Use assert_array_almost_equal from numpy.ma

This does not raise warnings about NaNs in the masked part of the
array.
Use ma.testutils rather than a filter for warnings with NaN in
masked arrays in test_mlab
Merge pull request #3244 from jenshnielsen/rcwarnings

Filter warnings in rcparams test (and others)