[tests] Re-enable disabled test.

 - This was failing because of non-determinism in the output due to
   the race conditions fixed in the last two commits. I verified the
   output is now stable over 100s of iterations on 14.04 (where it
   appeared most likely to fail).
diff --git a/tests/Examples/buildsystem-capi.llbuild b/tests/Examples/buildsystem-capi.llbuild
index 220af46..c9942d6 100644
--- a/tests/Examples/buildsystem-capi.llbuild
+++ b/tests/Examples/buildsystem-capi.llbuild
@@ -14,11 +14,6 @@
 # CHECK: command_finished: <fancy-thing>
 # CHECK: command_started: <error> -- FAILING-COMMAND
 # CHECK: <unknown>:0: error: build had 1 command failure
-#
-# FIXME: Temporarily disabled, this test sometimes fails to print the SourceMgr
-# issued diagnostic (the final error) during tests on 14.04.
-#
-# REQUIRES: disabled
 
 client:
   name: basic