fix linter issue

Author: asmyasnikov

tests/integration/helpers_test.go

@@ -473,7 +473,7 @@ func driverEngine(db *sql.DB) (engine xsql.Engine) {
 func simpleDetectGoroutineLeak(t *testing.T) {
 	// 1) testing.go => main.main()
 	// 2) current test
-	var expectedGoroutinesCount = 2
+	const expectedGoroutinesCount = 2
 	if num := runtime.NumGoroutine(); num > expectedGoroutinesCount {
 		bb := make([]byte, 2<<32)
 		if n := runtime.Stack(bb, true); n < len(bb) {