[Infra] Fix flaky 'NSURLSession+GULPromises' test (approach 2)

[ncooke3]

This is another approach to #119 that I think is probably better.

Some context, the test crashes are originating in tests that test our category on NSURLSession. We use OCMock to stub the behavior of the underlying NSURLSession. Rather than using OCMock, I figured we could use the real NSURLSession, and instead make local requests on file system URLs. This approach seems to actually work well and wasn't difficult to set up.

Fixes CI failures in PRs like #110 and #118

[ncooke3]

Hmm, that's weird, google-utilities / catalyst (pull_request) still failed. It seems the test crashing us unrelated to the test's implementation.

[google-oss-bot]

Coverage Report ¹

Affected Products

• GoogleUtilities-ios-unit-GoogleUtilities.framework

  Overall coverage changed from 0.62% (c38ce36) to 0.62% (470964c) by -0.00%.

  Filename	Base (c38ce36)	Merge (470964c)	Diff
  GULAppEnvironmentUtil.m	2.58%	3.14%	+0.57%
  IsAppEncrypted.m	?	0.00%	?

Test Logs

• Base (c38ce36): https://github.com/google/GoogleUtilities/actions/runs/5683012891
• Merge (470964c): https://github.com/google/GoogleUtilities/actions/runs/5787253176

1. https://storage.googleapis.com/firebase-sdk-metric-reports/FsRb7JYbIu.html

[ncooke3]

Pausing investigation into the crash. Weirdly, the promise timeout in this PR and in #119 is the same. The crash can be more easily produced by running the tests on Catalyst (via CocoaPods) and enabling the address sanitizer.