Flaky test: TestHotReloadUIConfigTempFile

[albertteoh]

Describe the bug
The TestHotReloadUIConfigTempFile test appears to fail due to timeout after not detecting the config changes.

To Reproduce
Steps to reproduce the behavior:

1. go clean -testcache && go test -test.count=50 ./cmd/query/app -v -run TestHotReloadUIConfigTempFile
2. Resulting failure (note the first succeeds, next attempt fails):

=== RUN   TestHotReloadUIConfigTempFile
--- PASS: TestHotReloadUIConfigTempFile (0.13s)
=== RUN   TestHotReloadUIConfigTempFile
    static_handler_test.go:161: 
                Error Trace:    static_handler_test.go:161
                Error:          timed out waiting for the hot reload to kick in
                Test:           TestHotReloadUIConfigTempFile
--- FAIL: TestHotReloadUIConfigTempFile (1.04s)

Version (please complete the following information):

• OS: MacOS
• Jaeger version: master

What troubleshooting steps did you try?
Increasing the wait time to time.After(30 * time.Second) blocks for the entire 30 seconds before failing.

Additional context
Issue was observed while testing PR #2707.