Increase build context test limit

Summary:
The phabricator messaging code in particular doesn't use the count, and assumes the test list
is complete, which means that limiting tests will be potentially confusing and wrong.
This bumps us to a large enough limit that if we're limiting you, it's probably not worth
looking at individual tests anyway while still avoiding some pathological scenarios.

More complete fixes to come later.

Test Plan: None

Reviewers: paulruan

Reviewed By: paulruan

Subscribers: changesbot, anupc

Differential Revision: https://tails.corp.dropbox.com/D232071

changes/lib/build_context_lib.py

@@ -185,7 +185,7 @@ def _get_build_context(build, get_parent=True):
 
 def _get_job_context(job):
     # type: (Job) -> Dict[str, Any]
-    def get_job_failing_tests(job, limit=30):
+    def get_job_failing_tests(job, limit=500):
         failing_tests = TestCase.query.options(
             subqueryload_all('messages')
         ).filter(