Add an intercepting cabal-testsuite/cabal.project

- Reduce duplication in test flag setup
- Avoid "New config" picking cabal's own project

Before this change, this was the test failure:

Unit Tests
  UnitTests.Distribution.Client.Configure
    Configure tests
      New config:                     FAIL
        Exception: project-cabal/pkgs/cabal.config: withBinaryFile: does not exist (No such file or directory)
        Use -p '/New config/' to rerun this test only.
      Replacement + new config:       OK
      Old + new config:               OK
      Old + new config, no appending: OK
      Old + new config, backup check: OK
      Local program options:          OK

1 out of 6 tests failed (0.02s)