MDEV-33616 Tests failing on macOS

These tests rely on THR_KEY_mysys but it is not initialized.  On
Linux, the corresponding thread variable is null, but on macOS it has a
nonzero value.  In all cases, initialize the variable explicitly by
calling MY_INIT and my_end appropriately.

unittest/mysys/dynstring-t.c

@@ -26,8 +26,10 @@ static void check(const char *res)
   str1.length= 0;
 }
 
-int main(void)
+int main(int argc, char** argv)
 {
+  MY_INIT(argv[0]);
+
   plan(23);
 
   IF_WIN(skip_all("Test of POSIX shell escaping rules, not for CMD.EXE\n"), );
@@ -69,6 +71,7 @@ int main(void)
 
   dynstr_free(&str1);
 
+  my_end(MY_CHECK_ERROR);
   return exit_status();
 }
 

unittest/sql/my_json_writer-t.cc

@@ -51,6 +51,8 @@ class THD
 
 int main(int args, char **argv)
 {
+  MY_INIT(argv[0]);
+
   plan(NO_PLAN);
   diag("Testing Json_writer checks");
 
@@ -123,6 +125,7 @@ int main(int args, char **argv)
 
   diag("Done");
 
+  my_end(MY_CHECK_ERROR);
   return exit_status();
 }