spanner: use emulator by default when running integration tests with Spanner (#102757)

* spanner: when using GRAFANA_TEST_DB=spanner and no SPANNER_DB, use emulator by default.

pkg/services/sqlstore/sqlutil/sqlutil.go

@@ -161,7 +161,7 @@ func spannerTestDB() (*TestDB, error) {
 
 	spannerDB := os.Getenv("SPANNER_DB")
 	if spannerDB == "" {
-		return nil, errors.New("SPANNER_DB environment variable not set")
+		spannerDB = "emulator"
 	}
 
 	if spannerDB == "spannertest" {