TST fix still broken test_fast_dot

On Travis, SciPy seems to raise AttributeError instead of ValueError.
Add a estimator introspection check to test_common
ENH make sure all "SkipTest" calls have an error message.
don't import tests
FIX don't raise file-level SkipTests
Merge pull request #2538 from ogrisel/test-test-common

[MRG] Investigate test_common non-running on travis
Merge pull request #2536 from larsmans/rm-mlcomp-doc-class

MAINT remove mlcomp document classif. example
ENH NMF estimator based on the Lee and Seung algorithm