spanner/spannertest: document using SPANNER_EMULATOR_HOST

Updates #1181.

Change-Id: I369b2cbadff6f7434ea79e878a57b186359f47cd
Reviewed-on: https://code-review.googlesource.com/c/gocloud/+/43231
Reviewed-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Jean de Klerk <deklerk@google.com>

spanner/spannertest/inmem.go

@@ -31,6 +31,14 @@ create a Server, and then connect to it with no security:
 	client, err := spanner.NewClient(ctx, db, option.WithGRPCConn(conn))
 	...
 
+Alternatively, create a Server, then set the SPANNER_EMULATOR_HOST environment
+variable and use the regular spanner.NewClient:
+	srv, err := spannertest.NewServer("localhost:0")
+	...
+	os.Setenv("SPANNER_EMULATOR_HOST", srv.Addr)
+	client, err := spanner.NewClient(ctx, db)
+	...
+
 The same server also supports database admin operations for use with
 the cloud.google.com/go/spanner/admin/database/apiv1 package.
 */

spanner/spannertest/integration_test.go

@@ -69,11 +69,8 @@ func makeClient(t *testing.T) (*spanner.Client, *dbadmin.DatabaseAdminClient, fu
 		return client, adminClient, func() { client.Close(); adminClient.Close() }
 	}
 
-	// Unfortunately the Spanner client library does not permit insecure connections
-	// like other client libraries (providing an endpoint and grpc.WithInsecure fails
-	// because it uses transport.DialGRPC directly), so we need to manually construct the
-	// gRPC connection and provide it to the client library. This may bypass our ability
-	// to check some aspects of the connection pooling.
+	// Don't use SPANNER_EMULATOR_HOST because we need the raw connection for
+	// the database admin client anyway.
 
 	t.Logf("Using in-memory fake Spanner DB")
 	srv, err := NewServer("localhost:0")