The idea here is to pass a `--package-db` flag to `cabal-install` which
contains just `Cabal` and `Cabal-syntax` of the specific version. This
allows `cabal-install` tests to use the in-tree `Cabal` version,
something which you can easily run into and get very confused about when
writing tests.

There are a few options which can be passed to `cabal-tests` executable
to control which Cabal library you will test against.

1. --boot-cabal-lib specifies to use the Cabal library bundled with the
   test compiler, this is the default and existing behaviour of the
   testsuite.
2. --intree-cabal-lib=<root_dir> specifies to use Cabal and Cabal-syntax
   from a specific directory, and `--test-tmp` indicates where to put
   the package database they are built with.
3. --specific-cabal-lib=<VERSION> specifies to use a specific Cabal
   version from hackage (ie 3.10.2.0) and installs the package database
   into --test-tmp=<DIR>

The end result is that changes in the Cabal library can be tested with
cabal-install tests in the testsuite.

There have been a number of confusing issues with people writing tests
for changes in the Cabal library which never ran because of
cabal-install tests always used the boot Cabal library (see #9425
for one).

Fixes #9681