Add unit tests for the solver's "rejecting" message with multiple versions

[sebright]

Tests of the solver's current "rejecting" message would help us track progress on #4251. These tests could go in cabal-install/tests/UnitTests/Distribution/Solver/Modular/Solver.hs, where we already have tests for other messages, such as

cabal/cabal-install/tests/UnitTests/Distribution/Solver/Modular/Solver.hs

Lines 603 to 634 in d958545

	[ -- Skipping a version because of a problematic dependency:
	--
	-- When the solver explores A-4, it finds that it cannot satisfy B's
	-- dependencies. This allows the solver to skip the subsequent
	-- versions of A that also depend on B.
	runTest $
	let db =
	[ Right $ exAv "A" 4 [ExAny "B"]
	, Right $ exAv "A" 3 [ExAny "B"]
	, Right $ exAv "A" 2 [ExAny "B"]
	, Right $ exAv "A" 1 []
	, Right $ exAv "B" 2 [ExAny "unknown1"]
	, Right $ exAv "B" 1 [ExAny "unknown2"]
	]
	msg =
	[ "[__0] trying: A-4.0.0 (user goal)"
	, "[__1] trying: B-2.0.0 (dependency of A)"
	, "[__2] unknown package: unknown1 (dependency of B)"
	, "[__2] fail (backjumping, conflict set: B, unknown1)"
	, "[__1] trying: B-1.0.0"
	, "[__2] unknown package: unknown2 (dependency of B)"
	, "[__2] fail (backjumping, conflict set: B, unknown2)"
	, "[__1] fail (backjumping, conflict set: A, B, unknown1, unknown2)"
	, "[__0] skipping: A; 3.0.0, 2.0.0 (has the same characteristics that "
	++ "caused the previous version to fail: depends on 'B')"
	, "[__0] trying: A-1.0.0"
	, "[__1] done"
	]
	in setVerbose $
	mkTest db "skip version due to problematic dependency" ["A"] $
	SolverResult (isInfixOf msg) $
	Right [("A", 1)]

Here are some cases to test:

• Rejecting multiple versions
• Rejecting versions that aren't in order, using preferences
• Rejecting source and installed versions
• Rejecting linked instances, using setup or build-tool dependencies

[erikd]

Is the unit test added in #11062 sufficient?