sonic_data_client: make PollStats stoppable and wire into TestMain

[sigabrtv1-ui]

Problem

PollStats() is started in sonic_data_client's init() and never stopped. The goroutine leaks across test boundaries in both gnmi_server and sonic_data_client test packages.

When tests use gomonkey to patch os.OpenFile or os.File methods, the still-running PollStats goroutine calls linuxproc.ReadStat("/proc/stat") which goes through os.OpenFile, hits the mock, and receives a zero-value *os.File{}. Calling Stat() on that causes a nil pointer dereference:

panic: runtime error: invalid memory address or nil pointer dereference
goroutine in PollStats() → linuxproc.ReadStat → os.ReadFile → os.(*File).Stat

Also observed as a DATA RACE in CI build 20260305.14.

Fix

sonic_data_client/non_db_client.go

• Add pollStatsDone chan struct{} + sync.Once
• Add exported StopPollStats() — safe to call multiple times
• PollStats() selects on done channel each iteration and exits when closed
• Use select { case <-time.After(...) } instead of time.Sleep for immediate shutdown response

gnmi_server/server_test.go and sonic_data_client/client_test.go

• Call StopPollStats() in TestMain before running tests in both packages

Notes

• StopPollStats() is intentionally test-only — production code never calls it
• Calling it multiple times is safe (sync.Once)
• This is a prerequisite for PR Makefile: add -gcflags=all=-l to gnmi_server tests for gomonkey compatibility #604 (-gcflags=all=-l): once gomonkey patches are effective, this goroutine would otherwise race and crash the test binary

[mssonicbld]

/azp run

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[hdwhdw]

/azp run

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