patch 8.1.2395: using old C style comments

Problem:    Using old C style comments.
Solution:   Use // comments where appropriate.
diff --git a/src/testing.c b/src/testing.c
index 7543dd8..f879f1e 100644
--- a/src/testing.c
+++ b/src/testing.c
@@ -813,8 +813,8 @@
     void
 f_test_garbagecollect_now(typval_T *argvars UNUSED, typval_T *rettv UNUSED)
 {
-    /* This is dangerous, any Lists and Dicts used internally may be freed
-     * while still in use. */
+    // This is dangerous, any Lists and Dicts used internally may be freed
+    // while still in use.
     garbage_collect(TRUE);
 }