Allow changing tested-with in revisions (redux)

[ulysses4ever]

previously reported in #583 and allegedly fixed in #598 but it doesn't actually look like it was fixed because of this line:

hackage-server/Distribution/Server/Util/CabalRevisions.hs

Line 257 in 31bcb03

changesOk' Trivial "tested-with" (show . ppTestedWith) testedWithA testedWithB

suggests that tested-with is still a trivial change and, hence, won't be alowed per

hackage-server/Distribution/Server/Util/CabalRevisions.hs

Lines 76 to 78 in 31bcb03

	| foldMap changeSeverity changes /= Trivial -> Right changes
	| otherwise ->
	Left "Only trivial changes, don't bother making this revision."