BUG: Fix multivariate_normal issue with 'size' argument

Ensure that the multivariate_normal size argument can be a numpy integer. Add regression test. Apply PEP8 to test_regression.py. Regenerate mtrand.c.
TST: remove docstrings from test_ functions

See doc/TESTS.rst.txt for the reason.
Merge pull request #3373 from cgohlke/multivariate_normal-fix

multivariate_normal issue with 'size' argument