create separate test file for lib.mdamath

[orbeckst]

Is your feature request related to a problem?

WHen looking at the tests in testsuite/MDAnalysisTests/lib I'd expect to find one test file for each of the major sub modules. But there's no test_mdamath.py. Instead there's a humongously big test_utils.py, which makes it harder to add new tests.

Describe the solution you'd like

Break test_utils.py into smaller files, in particular create test_mdamath.py with anything from lib.mdamath

Describe alternatives you've considered

do nothing, it works

Additional context