Fix broken samples API

The samples API was performing an invalid SQL query since the ORM
update in 5b02472. This fixes the problem and un-XFAILs the test.

Author: ldionne

lnt/server/ui/api.py

@@ -438,6 +438,7 @@ def get(machine_id, test_id, field_index):
 
         q = session.query(field.column, ts.Order.llvm_project_revision,
                           ts.Run.start_time, ts.Run.id) \
+            .select_from(ts.Sample) \
             .join(ts.Run) \
             .join(ts.Order) \
             .filter(ts.Run.machine_id == machine.id) \

tests/server/ui/test_api.py

@@ -1,5 +1,3 @@
-# XFAIL: TODO-FIXME
-
 # Check that the LNT REST JSON API is working.
 # create temporary instance
 # RUN: rm -rf %t.instance