bug 1568889: tools/lint: run rustfmt on geckodriver and webdriver; r=webdriver-reviewers,whimboo

Includes geckodriver and the webdriver crate in the rustfmt job
on Treeherder.

Enabling this does not cause any errors, but we are seeing 93 warnings.
These are not fatal and do not cause the job to fail in continuous
integration.

Differential Revision: https://phabricator.services.mozilla.com/D39368

--HG--
extra : moz-landing-system : lando

tools/lint/rustfmt.yml

@@ -3,6 +3,8 @@ rust:
     description: Reformat rust
     include:
         - js/src/wasm/cranelift/
+        - testing/geckodriver/
+        - testing/webdriver/
     extensions:
         - rs
     support-files: