Limit test results provided to email templates

Summary:
This is mostly a self-defense measure, to keep from potentially processing
thousands and thousands of tests.
If users want more than a handful of tests, they can go to the UI.
Since we also report the test count, the risk of users assuming they're seeing
the complete failed picture is reduced.

Test Plan: Existing

Reviewers: paulruan

Reviewed By: paulruan

Subscribers: anupc

Differential Revision: https://tails.corp.dropbox.com/D232011

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):
+    def get_job_failing_tests(job, limit=30):
         failing_tests = TestCase.query.options(
             subqueryload_all('messages')
         ).filter(
@@ -198,7 +198,7 @@ def get_job_failing_tests(job):
                 'test_case': test_case,
                 'uri': build_web_uri(_get_test_case_uri(test_case)),
                 'message': xunit.get_testcase_messages(test_case),
-            } for test_case in failing_tests
+            } for test_case in failing_tests[:limit]
         ]
         failing_tests_count = len(failing_tests)