`cabal test` should display test execution environment

[ttuegel]

As noted in #2058, cabal test uses showCommandForUser, but what we really want is showCommandEnvForUser.

[psibi]

@ttuegel Is showCommandEnvForUser already implemented ? Also I don't see cabal test using showCommandForUser using anywhere. The testAction method doesn't seem to be implemented using it.

[ttuegel]

Actually, this is already fixed and I forgot to close it. cabal test runs the executable with rawSystemIOWithEnv which prints the command and environment.