Remove parametric testing machinery
Remove tests for parametric test machinery
Remove most uses of ParametricTestCase
Remove ParametricTestCase from nbconvert tests
Remove uses of @parametric decorator
Fix calls to validate_message
Remove workaround for parametric test weirdness
move kernel-test utilities to kernel.tests.utils

context managers and tools for starting kernels,
and sharing a *single* global kernel for many tests.

reduce the number of kernels started for IPython.kernel tests,
which should save some time.
add missing `now` to MKM.restart
use forceful restart / shutdown in KM / MKM tests