nobacktick test: Use literal backtick in test.

With proper quoting, the non-POSIX \d96 escape isn't necessary, and the check should work also on Mac OS.

tests/t0100-code-nobacktick.sh

@@ -5,11 +5,12 @@ test_description='no old-style backtick command substitution
 This test checks the todo.sh script itself for occurrences
 of old-style backtick command substitution, which should be
 replaced with $(...).
+On failure, it will print each offending line number and line.
 '
 . ./test-lib.sh
 
 test_todo_session 'no old-style backtick command substitution' <<EOF
->>> sed -n -e 's/[ \t]#.*//' -e '/\d96/{=;p}' "$(which todo.sh)"
+>>> sed -n -e 's/\(^\|[ \t]\)#.*//' -e '/\`/{' -e '=;p' -e '}' "$(which todo.sh)"
 EOF
 
 test_done