tests: silence "referenced but not assigned" warnings

The optstring* helpers do the assignment, which isn't particularly easy
for shellcheck to deduce. So silence the warnings.

Signed-off-by: Emil Velikov <[email protected]>

Author: evelikov

test/test_option_twiddling

@@ -26,8 +26,10 @@ EXPECT_success optstring_has_option optstring maxproto
 
 EXPECT_failure optstring_get_option optstring proto
 EXPECT_success optstring_get_option optstring maxproto
+# shellcheck disable=SC2154 # set via the optstring helper above
 ASSERT_streq "$maxproto" "5"
 EXPECT_success optstring_get_option optstring timeout
+# shellcheck disable=SC2154 # set via the optstring helper above
 ASSERT_streq "$timeout" "300"
 
 optstring_remove_option optstring pgrp