Deduplicate config file path calculation in test.

[athas]

Tested with cabal test Cabal-tests:hackage-tests (man does Cabal have many test suites).

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[phadej]

Now looking at this in own context i see that this is not good. defaultConfigFile doesn't consider $CABAL_CONFIG environment variable, so that is wrong.

Perfectly there should be a function in cabal-install to find the config file (c.f. https://hackage.haskell.org/package/cabal-install-parsers-0.4.1/docs/src/Cabal.Config.html#findConfig) but AFAICS it's backed into loadRawConfig (which is not bad, you probably want to read the config - as in here).

Yet, adding cabal-install lib as dependency to Cabal-tests doesn't feel right. There is "configuration parsing library" screaming to get out, whole cabal-install is big.

So, either use loadRawConfig to not duplicate any parsing logic, or let's drop this change.

[athas]

Now looking at this in own context i see that this is not good. defaultConfigFile doesn't consider $CABAL_CONFIG environment variable, so that is wrong.

defaultConfigFile uses getCabalDir, which looks up the CABAL_DIR environment variable AFAICS.

I guess one could use loadRawConfig, but the types involved are completely different, so I have no idea what that would actually entail.

[athas]

Oh, right, CABAL_CONFIG... well, is that needed for this test? I'm not even completely sure what it is testing.

[athas]

Now it uses getConfigFilePath, which respects CABAL_CONFIG. I don't see a realistic way to get rid of the cabal-install dependency.

[phadej]

I'm not even completely sure what it is testing.

hackage-tests tests that whole Hackage's 01-index.tar can be parsed (when we do .cabal file parser changes). So it has to find 01-index.tar.

[Mikolaj]

@athas: do you need help with this PR or a future incarnation of it? Please shout to me, if so.

[athas]

I kind of lost motivation to do anything with it, since I don't fully understand the problem or the solution. Maybe it will become necessary to address as I do some more work on #7386 (particularly when I implement the CABAL_HOME fallback behaviour).