Previously this would work until CABAL_TESTSUITE_BDIR was needed, at
which point it would fail because it looked for things under
.../ghc-8.6/... while cabal v2-build actually put them in
.../ghc-8.6.5/...

Hard-coding "ghc" is unfortunate, but I didn't know if there was an easy
way to avoid it.  I also didn't want to over-complicate it, until it was
necessary.

validate.sh isn't tested in CI so...
[ci skip]

In recent versions of cabal-plan the --builddir= flag has become a
non-global flag; i.e. it needs to be placed after the sub-command rather
than before.

`validate.sh` isn't tested in CI so...
[ci skip]

- Add bunch of installed constraints to cabal.project.local.travis
- Fix compat in D.Compat.Directory
- Compat (process-1.1/ghc-7.6) in cabal-testsuite
- validate.sh improvements
- Warningless cabal-testsuite build

Resolve #5025

Now there's --help, so it's easier to use :)

This script contains similar commands to appveyor.yml, except it relies
on cabal new-build, so should be safe to run locally.

Using this script can ease Travis stress.

ATM I see following tests failing:

- PackageTests/Backpack/Includes3/setup-external-ok.test.hs
    - https://github.com/haskell/cabal/issues/5039
- PackageTests/CustomPlain/setup.test.hs
    - `<command line>: cannot satisfy -package-id Cabal-2.1.0.0-inplace`
- PackageTests/CustomDep/sandbox.test.hs
    - `.../dist-sandbox-3402c05b/setup/setup: createProcess: runInteractiveProcess: exec: permission denied (Permission denied)`