patch 8.2.0978: leaking memory in termcodes test

Problem:    Leaking memory in termcodes test.
Solution:   Set t_8u with set_option_value().
diff --git a/src/term.c b/src/term.c
index b806a89..f8b5659 100644
--- a/src/term.c
+++ b/src/term.c
@@ -4690,7 +4690,7 @@
 	// Unless the underline RGB color is expected to work, disable "t_8u".
 	// It does not work for the real Xterm, it resets the background color.
 	if (term_props[TPR_UNDERLINE_RGB].tpr_status != TPR_YES && *T_8U != NUL)
-	    T_8U = empty_option;
+	    set_option_value((char_u *)"t_8u", 0L, (char_u *)"", 0);
 
 	// Only set 'ttymouse' automatically if it was not set
 	// by the user already.