integration_test: Add gitignore of golden image (#161404)

Ignore the golden file associated with

packages/integration_test/example/integration_test/integration_test_matches_golden_file.png

Test (and functionality) introduced in #160484.

Related issues:
* https://github.com/flutter/flutter/issues/143299.
* https://github.com/flutter/flutter/issues/160043.

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packages/integration_test/example/.gitignore

@@ -44,3 +44,6 @@ app.*.map.json
 /android/app/debug
 /android/app/profile
 /android/app/release
+
+# Golden images.
+integration_test/integration_test_matches_golden_file.png