Update gitignore for integration test files

[a-maurice]

Description

Provide details of the change, and generalize the change in the PR title above.

Add files that are generated when building the integration tests to the gitignore file

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Testing

Describe how you've tested these changes. Link any manually triggered Integration tests or CPP binary SDK Packaging Github Action workflows, if applicable.

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Type of Change

Place an x the applicable box:

• Bug fix. Add the issue # below if applicable.
• New feature. A non-breaking change which adds functionality.
• Other, such as a build process or documentation change.

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Notes

• Bug fixes and feature changes require an update to the Release Notes section of release_build_files/readme.md.
• Read the contribution guidelines CONTRIBUTING.md.
• Changes to the public API require an internal API review. If you'd like to help us make Firebase APIs better, please propose your change in a feature request so that we can discuss it together.

[AlmostMatt]

This seems ok, but I also kind of wish that these autogenerated source files and folders were more clearly marked as such. For example, if they all ended up in a subdirectory called integration_test_build

[github-actions]

❌  Integration test FAILED

Requested by @a-maurice on commit 09ac17f
Last updated: Fri Oct 21 19:35 PDT 2022
View integration test log & download artifacts

Failures	Configs
missing_log	[TEST] [ERROR] [Android] [All 3 os] [1/2 android_device: android_target]
gma	[TEST] [FAILURE] [iOS] [macos] [1/2 ios_device: ios_target] (1 failed tests)   FirebaseGmaTest.TestRewardedAdStress
storage	[TEST] [ERROR] [iOS] [macos] [All 2 ios_device] [TEST] [ERROR] [tvOS] [macos] [tvos_simulator]

Add flaky tests to go/fpl-cpp-flake-tracker