Add //third_party/freetype-testing/src to .gitignore

This directory is pulled via DEPS since https://crrev.com/c/2529104
and causes git status to report that the checkout contains untracked
files.

Bug: 1140584
Change-Id: Ie837a23d6b4d6f908a70db9f0e03421ce28cf291
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2534953
Auto-Submit: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Dominik Röttsches <drott@chromium.org>
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#826825}

third_party/.gitignore

@@ -78,6 +78,7 @@
 /flac
 /flatbuffers/src
 /fontconfig/src
+/freetype-testing/src
 /freetype/src
 /fuchsia-sdk/images
 /fuchsia-sdk/images-internal