Mark VerifyNone as a test Helper

[tallclair]

Mark VerifyNone(...) as a test Helper function, so that it reports the error at the call site, rather than leaks.go:78:.

From the go docs:

Helper marks the calling function as a test helper function. When printing file and line information, that function will be skipped. Helper may be called simultaneously from multiple goroutines.

This is useful for tests that verify go routines at multiple points, so failures can be more easily distinguished.

[CLAassistant]

All committers have signed the CLA.

[abhinav]

Thanks for the contribution, @tallclair!
This is a reasonable request, but we'd prefer to do this in a backwards compatible way.
I've left a suggestion on how we can do that.

Separately, can you please also sign the CLA? We cannot land the change without that.

[tallclair]

I'm waiting on approval from my org to sign the CLA, sorry I may have jumped the gun on this.

[tallclair]

Sorry for the delay. CLA is signed, and comments have been addressed.

[abhinav]

Thanks!