use synctest to make streams map tests fully deterministic (#5296)

* use synctest to make streams map tests fully deterministic

* ignore synctest helper package for code coverage

internal/synctest/synctest_go124.go

@@ -12,3 +12,7 @@ func Test(t *testing.T, f func(t *testing.T)) {
 		f(t)
 	})
 }
+
+func Wait() {
+	synctest.Wait()
+}

internal/synctest/synctest_go125.go

@@ -10,3 +10,7 @@ import (
 func Test(t *testing.T, f func(t *testing.T)) {
 	synctest.Test(t, f)
 }
+
+func Wait() {
+	synctest.Wait()
+}