Set mutation covered percentage threshold in tests to 25

[jonasfreimuth]

This leads to enough samples being used that the mutation regression will run, meaning now most analysis code actually runs now during testing. The results may be unrealistic, but we don't care about that during testing.

[jonasfreimuth]

Changed to 25%, as 20% can lead to inconsistencies between differnt data loading due to a bug when loading mutations files where one column is always "T", which would have been read in as "TRUE" when using read.table (and wrappers).

[rekado]

This looks good to me, though I'd prefer to have a comment in the settings file. Better to have it where we can see it instead of just the git log.

[jonasfreimuth]

Will do.