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 https://github.com/haskell/cabal/blob/d958545e9cc745aef2db467d21f0802fee3f158e/cabal-install/tests/UnitTests/Distribution/Solver/Modular/Solver.hs#L603-L634

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