refs #9895 This should fix ConvertToDiffractionMDWorkspace tests

The test is not written by me but just decreasing number of test points by two orders of magnitude do works without, seems, affecting the coverage.
refs #9895 This should fix MaskPeaksWorkspaceTest

by creating source workspace with the same number of events as before.
Merge remote-tracking branch 'origin/bugfix/9895_SlowUT'