Only include relevant tests in a run report.

 - This limits the set of test names we consider (and thus report on) to be
   tests that appeared in any of the loaded runs. This fixes the appearance of
   spurious tests in places that look at all of the test names, for example the
   textual output from the 'lnt submit' command line tool.

 - Also, fix RunInfo.get_test_ids() to not return duplicates.

git-svn-id: https://llvm.org/svn/llvm-project/lnt/trunk@189856 91177308-0d34-0410-b5e6-96231b3b80d8