Add lint for backticks in shell scripts

The "$(some_command arg1 arg2)" form is preferred to the
`some_command arg1 arg2` form because it nests unambiguously.
Add a lint for this to tidy.

python/tidy/servo_tidy_tests/shell_tidy.sh

@@ -4,4 +4,6 @@
 
 set -o nounset
 
+# Talking about some `concept in backticks` # shouldn't trigger
 echo "hello world"
+some_var=`echo "command substitution"`

python/tidy/servo_tidy_tests/test_tidy.py

@@ -52,6 +52,7 @@ class CheckTidiness(unittest.TestCase):
         errors = tidy.collect_errors_for_files(iterFile('shell_tidy.sh'), [], [tidy.check_shell], print_text=False)
         self.assertEqual('script does not have shebang "#!/usr/bin/env bash"', errors.next()[2])
         self.assertEqual('script is missing options "set -o errexit", "set -o pipefail"', errors.next()[2])
+        self.assertEqual('script should not use backticks for command substitution', errors.next()[2])
         self.assertNoMoreErrors(errors)
 
     def test_rust(self):