Fix newlines between white/blacklist test names in buildkite annotations (#1003)

show-expected-fail-tests.sh

@@ -80,8 +80,8 @@ done <<< "${passed_but_expected_fail}"
 # TODO: Check that the same test doesn't appear twice in the whitelist|blacklist
 
 # Trim test output strings
-tests_to_add=$(echo -e $tests_to_add | xargs -d '\n')
-already_in_whitelist=$(echo -e $already_in_whitelist | xargs -d '\n')
+tests_to_add=$(IFS=$'\n' echo "${tests_to_add[*]%%'\n'}")
+already_in_whitelist=$(IFS=$'\n' echo "${already_in_whitelist[*]%%'\n'}")
 
 # Format output with markdown for buildkite annotation rendering purposes
 if [ -n "${tests_to_add}" ] && [ -n "${already_in_whitelist}" ]; then