gNMI connection: Avoid t.Fatalf in goroutine and ensure concurrency tested.
gNMI connection: Avoid t.Fatalf in goroutine and ensure concurrency tested.

Remove t.Fatalf call from TestConcurrentDialErr goroutine, and refactor to
ensure all goroutines are started before initiating dials.

Addresses the issue originally raised by @alrs in https://github.com/openconfig/gnmi/pull/66.

PiperOrigin-RevId: 295801877

openconfig/gnmi,openconfig/gnmi,openconfig/gnmi