Convert tests to use Truth

DocServletTest is still failing, but the failure message is now more
descriptive.

Change-Id: I9e84767c4d091d46abada4ab059aea2d4643cdba

gitiles-servlet/BUCK

diff --git a/gitiles-servlet/BUCK b/gitiles-servlet/BUCK
index 9e7a1e8..6e89181 100644
--- a/gitiles-servlet/BUCK
+++ b/gitiles-servlet/BUCK
@@ -36,6 +36,7 @@
     ':servlet',
     # Gitiles is designed to compile against the Servlet API v2.5.
     '//lib:servlet-api_2_5',
+    '//lib:truth',
     '//lib/jgit:junit',
     '//lib/junit:junit',
   ],