Use lateinit var to remove !! and created a common TestFileUtils

[phaiting]

Summary

This change did two changes:

• Use lateinit var to remove !!
• Created a common TestFileUtils to share the same code for getTestFilePath across all instrumentation test code.

Test plan

./gradlew :executorch_android:connectedAndroidTest

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/11155

• 📄 Preview Python docs built from this PR

Note: Links to docs will display an error until the docs builds have been completed.

✅ You can merge normally! (2 Unrelated Failures)

As of commit 69eb83d with merge base 0cf2d53 ():

BROKEN TRUNK - The following jobs failed but were present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• pull / unittest / linux / linux-job (gh) (trunk failure)
  examples/models/llama/tests/test_export_llama_lib.py::ExportLlamaLibTest::test_has_expected_ops_and_op_counts
• pull / unittest-editable / linux / linux-job (gh) (trunk failure)
  examples/models/llama/tests/test_export_llama_lib.py::ExportLlamaLibTest::test_has_expected_ops_and_op_counts

This comment was automatically generated by Dr. CI and updates every 15 minutes.

[phaiting]

@pytorchbot label "release notes: none"