disable map_key_test temporarily

- disable this test until golang issue 38105&38940 is fixed

type_tests/map_key_test.go

@@ -1,5 +1,6 @@
-// +build !go1.14
-// remove these tests temporarily until https://github.com/golang/go/issues/39105 is fixed
+// +build go1.15
+// remove these tests temporarily until https://github.com/golang/go/issues/38105 and
+// https://github.com/golang/go/issues/38940 is fixed
 
 package test