Stop stackdriver unit tests from doing a real healthcheck

Running `StackdriverTraceAutoConfigurationTests` leaks environment by trying to actually do a health check.

Figure out how to turn off health check or replace with no-op.

```
Error when performing Stackdriver AsyncReporter health check.
io.grpc.StatusRuntimeException: PERMISSION_DENIED: The request is missing a valid API key.

```

or

```
WARN c.g.c.s.a.t.StackdriverTraceAutoConfiguration - Error when performing Stackdriver AsyncReporter health check.
java.lang.IllegalStateException: timeout waiting for onClose. timeoutMs=5000, resultSet=false
```