refactor: move tester navigation out of integration tests #2934

Job	Run time
flutter-format	1m 37s
bootstrap (ubuntu_bootstrap_test.dart)	6m 41s
flutter-mocks	5m 21s
bootstrap (screenshot_test.dart)	13m 45s
flutter-analyze	2m 19s
flutter-tests	7m 57s
Go: Code sanity	1m 50s
init (ubuntu_init_test.dart)	2m 56s
Go: Tests	2m 4s
init-e2e	4m 59s
init (screenshot_test.dart)	2m 27s
l10n	1m 36s
build-and-upload	0s
Convert and upload coverage reports for TiCS	0s
	53m 32s

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