testsuite: Pass pkgdb of store used for intree Cabal

[sheaf]

Template B: This PR does not modify behaviour or interface of Cabal or cabal-install – it is purely a change in the testsuite.

This PR modifies cabal-testsuite to allow passing the path to the "store" package database. This allows the test suite to handle building packages with a custom setup that depends on other packages (which get installed into the store).

Checklist:

• [x] Patch conforms to the coding conventions.
• [x] Add a test that requires this functionality.

[alt-romes]

This simple but relevant test suite change is ready to be reviewed.

In upcoming #9871, this change is tested by BuildToolPaths because process will be installed in the store when building the in-tree libraries and needed for the test to pass.