statsmodels:unit tests for mixture_rvs.py (#783)

 * Add 4 new unit test to functions: mixture_rvs, mv_mixture_rvs,
   mixture_pdf, mixture_cdf.
statsmodels:corrected some issues as PR comments (#783)

 * delete subclass TestCase.
 * rename 2 test with the suffix random and create 2 new test with a ramdom seed fixed.
 * delete an attribute not used in test_mv_mixture_rvs_random to set the distributions.
 * change pdf and cdf tests to check 10 values hard code instead checking mean and variance.
add missing blank line for code clarity
BUG: try to work around numpy bug in np.random.multivariate_normal