patch 9.0.1557: test failures for unreachable code

Problem:    Test failures for unreachable code.
Solution:   Add a test override to ignore unreachable code.
diff --git a/src/testing.c b/src/testing.c
index dfa9c60..fd55927 100644
--- a/src/testing.c
+++ b/src/testing.c
@@ -1039,6 +1039,8 @@
 	no_wait_return = val;
     else if (STRCMP(name, (char_u *)"ui_delay") == 0)
 	ui_delay_for_testing = val;
+    else if (STRCMP(name, (char_u *)"unreachable") == 0)
+	ignore_unreachable_code_for_testing = val;
     else if (STRCMP(name, (char_u *)"term_props") == 0)
 	reset_term_props_on_termresponse = val;
     else if (STRCMP(name, (char_u *)"vterm_title") == 0)