patch 8.2.4087: cannot test items from an autoload script easily

Problem:    Cannot test items from an autoload script easily.
Solution:   Add the "autoload" value for test_override().
diff --git a/src/testing.c b/src/testing.c
index eef5e0d..3482682 100644
--- a/src/testing.c
+++ b/src/testing.c
@@ -1055,6 +1055,8 @@
 	    reset_term_props_on_termresponse = val;
 	else if (STRCMP(name, (char_u *)"uptime") == 0)
 	    override_sysinfo_uptime = val;
+	else if (STRCMP(name, (char_u *)"autoload") == 0)
+	    override_autoload = val;
 	else if (STRCMP(name, (char_u *)"ALL") == 0)
 	{
 	    disable_char_avail_for_testing = FALSE;