Be clearer about when and why we override HttpClient in tests

[dnfield]

Description

Outside of browser contexts, we override the HttpClient implementation to one that always returns 400 - this helps make tests more hermetic and prevent users from making mistakes in tests by introduce real life network calls into a unit test.

This can be confusing for users, particularly if they're mixing both testWidgets and test tests in the same suite, and it's unclear that the binding has taken over the whole suite. This adds new functionality to print out a warning message if a failing test has created our special HttpClient, and updates docs on the TestFlutterWidgetsBinding class.

Related Issues

fixes #35318

Tests

I added the following tests:

Test that checks that we print out the message, and that we only do it once per suite.

Checklist

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• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I signed the CLA.
• I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I updated/added relevant documentation (doc comments with ///).
• All existing and new tests are passing.
• The analyzer (flutter analyze --flutter-repo) does not report any problems on my PR.
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Breaking Change

Did any tests fail when you ran them? Please read Handling breaking changes.

• No, no existing tests failed, so this is not a breaking change.

[jonahwilliams]

This would output the message for any widget test which incidentally built a network image. It doesn't seem desirable if their test was otherwise working.

[dnfield]

Is there some way to just do this on a test failure?

[dnfield]

I could try to use test_package.printOnFailure, but I'm not sure how to test that anymore - hah

[dnfield]

I guess I can put it in a new test file and run it under a harness that asserts the test fails and greps the message.

[dnfield]

Ok - now it should only print the warning if a test fails and the HttpClient was created.

[jonahwilliams]

LGTM if LG to everyone else

[Piinks]

LGTM!

[fluttergithubbot]

It looks like this pull request may not have tests. Please make sure to add tests before merging. If you need an exemption to this rule, contact Hixie.

Reviewers: Read the Tree Hygiene page and make sure this patch meets those guidelines before LGTMing.

[dnfield]

The bot fell asleep on the job!