Added masked version of iris.tests.IrisTest.assertArrayAllClose.

This change parallels the other assertMaskedXXX routines.
It enables me to fix the problem with the test code in lib/iris/tests/experimental/regrid/test_regrid_conservative_via_esmpy.py, which no longer works (i.e. the test code fails) with numpy v1.7.
( But I'll put that actual fix in an upcoming PR )

Note: this problem was not spotted earlier, as these tests are skipped when ESMPy is not installed 
 -- so, not in Travis.