Expand the .gitignore file

This adds several patterns to the `.gitignore` file to prevent accidentally committing files related to contributing through version control.

The `wp-test-runner` directory is also being added as the default directory for running tests locally.

Author: desrosj

.gitignore

@@ -3,4 +3,13 @@
 vendor/
 package-lock.json
 commit.json
-ignore.json
+ignore.json
+
+# Exclude the default test directory.
+wp-test-runner/
+
+# Files related to applying patches
+*.rej
+*.orig
+*.patch
+*.diff