[Task][Go SDK]: Update fhirio tests to not rely on direct runner.

### What needs to happen?

The tests rely on overriding the GCP FHIR client with a test interface double, however, it hasn't been done in a serializable form, relying on the Direct Runner to not serialize the DoFns.

eg. https://github.com/apache/beam/blob/master/sdks/go/pkg/beam/io/fhirio/deidentify.go#L85

https://github.com/apache/beam/blob/master/sdks/go/pkg/beam/io/fhirio/common.go#L205

This is not a good testing style as it isn't possible to execute them on portable runners, like prism, flink, spark, dataflow etc.

Since this requires a bit more thought and code to resolve, fhirio will continue to use the direct runner as the default for now, but should be changed to an alternative testing means (possibly by a local GRPC connection or similar to a local fake of the service, setting the client endpoint to be overridden, with an API dialing option at client creation https://pkg.go.dev/google.golang.org/api/option#WithEndpoint)

https://github.com/apache/beam/blob/master/sdks/go/pkg/beam/io/fhirio/common.go#L97

This is used for the native SpannerIO tests, as an example. https://github.com/apache/beam/blob/master/sdks/go/pkg/beam/io/spannerio/common.go#L53

### Issue Priority

Priority: 3 (nice-to-have improvement)

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