patch 9.0.0890: no test for what patch 9.0.0827 fixes

Problem:    No test for what patch 9.0.0827 fixes.
Solution:   Add a test (still doesn't fail when fix is reverted).
diff --git a/src/evalvars.c b/src/evalvars.c
index 132de4c..793f563 100644
--- a/src/evalvars.c
+++ b/src/evalvars.c
@@ -1343,6 +1343,11 @@
 
     if (*arg == '@' && arg[1] != NUL)
 	return arg + 2;
+
+    // termcap option name may have non-alpha characters
+    if (STRNCMP(arg, "&t_", 3) == 0 && arg[3] != NUL && arg[4] != NUL)
+	return arg + 5;
+
     end = find_name_end(*arg == '$' || *arg == '&' ? arg + 1 : arg,
 				   NULL, NULL, FNE_INCL_BR | FNE_CHECK_START);