Spanner-related fixes (#102376)

* Fix UNION syntax in resourcepermissions package.
* Fix migrations in usermig package to work with Spanner.
* Fix health query.
* Use more connections for integration tests.
* Add test-go-integration-spanner target to run integration tests against Spanner emulator.
* Add test for enterprise.
* Don't delete sequence number for migration_log.id column.

* Only bump max open connections to 20 for Spanner.
Lower integration test timeout.

pkg/api/health.go

@@ -15,7 +15,7 @@ func (hs *HTTPServer) databaseHealthy(ctx context.Context) bool {
 	}
 
 	err := hs.SQLStore.WithDbSession(ctx, func(session *db.Session) error {
-		_, err := session.Exec("SELECT 1")
+		_, err := session.Query("SELECT 1")
 		return err
 	})
 	healthy := err == nil