More consistently pass --package-db flag to tests

Previously `--package-db` was only passed to test which used the `v2-`
prefix.

Now we pass `--package-db` to things which use the `v2-` prefix, not
things which use the `v1-` prefix and by default assume that unprefixed
commands are v2 commands.

(cherry picked from commit 2b44677c)

# Conflicts:
#	cabal-testsuite/PackageTests/CustomTestCoverage/cabal.out