Add flaky test issue template (#8931)

Use this template for reporting flaky tests.

.github/ISSUE_TEMPLATE/flaky-test.md

@@ -0,0 +1,19 @@
+---
+name: Flaky Test
+about: Report a flaky test (one that doesn't pass consistently)
+
+---
+
+## Flaky Test
+
+* **Test Name:** Name of the failing test.
+* **Link:** Link to file/line number in github.
+* **Branch:** Git branch the test was seen in. If a PR, the branch the PR was based off.
+* **Artifact Link:** If available, attach the generated zip artifact associated with the stack trace for this failure.
+* **Notes:** Additional details about the test. e.g. theory as to failure cause
+
+### Stack Trace
+
+```
+paste stack trace here
+```