[path_provider] Add missing tests

[ened]

Description

This simply activates a integration test for a newly added method (getApplicationSupportDirectory) on the Android side.

Also completes the test suite for all path_provider methods.

Checklist

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• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• My PR includes unit or integration tests for all changed/updated/fixed behaviors (See Contributor Guide).
• All existing and new tests are passing.
• I updated/added relevant documentation (doc comments with ///).
• The analyzer (flutter analyze) does not report any problems on my PR.
• I read and followed the Flutter Style Guide.
• The title of the PR starts with the name of the plugin surrounded by square brackets, e.g. [shared_preferences]
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy.
• I updated CHANGELOG.md to add a description of the change.
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Breaking Change

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• Yes, this is a breaking change (please indicate a breaking change in CHANGELOG.md and increment major revision).
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Note

This integration test will fail until this PR is merged. I'm surprised this didn't block anywhere?

[ened]

Not good yet, I noticed a few test are missing.

[collinjackson]

Looks good I'll migrate this by hand to the new FirebaseExtended/flutterfire repo

[ened]

@collinjackson path_provider moves to FlutterFire?

[ened]

@collinjackson please have another look, there are platform unit tests for iOS as well.

Also, a recent commit (a3f9ce7) showed a potential issue that is not matched in the integration tests.

[collinjackson]

Looking at this again, I'm not thrilled with the function pathProviderPrivate at the global scope... I kinda wish that we'd made this plugin be a class instead of a bunch of individual functions. Do you think it might make sense to do that as a breaking change?

In any case, I think we should call it something more descriptive like setMockPathProviderPlatform

[ened]

I think the beauty of path_provider is the simplicity, thus would prefer to just add setMockPathProviderPlatform. Will update PR.

[ened]

@collinjackson ok - updated.

I think you may be right regarding a PathProvider class, but I would prefer to merge this PR, then #1953, #1716, maybe #1570 (if we have tests), and I may have another PR (based on #966) in preparation which enables multiple storage directories (as commonly seen on older Android phones w/ SD card and internal storage).

After that, version 2.0.0 could take over all of that and provide it nicely in a class interface.
WDYT?