Fix bugs in StationTestRun.AddFailure()

In addition, also fixes the following:
 - StationTestRun.AddArgument()
 - Only convert dargs to string while there is no primitive type
   mapping in JSON

TEST=make lint LINT_WHITELIST="py/test/testlog.py \
  py/test/testlog_utils.py py/goofy/invocation.py"
TEST=Manually

Change-Id: I797bff2bc4e832d6bfc8584212d802eadc7561ef
Reviewed-on: https://chromium-review.googlesource.com/361484
Commit-Ready: Joel Kitching <kitching@chromium.org>
Tested-by: Joel Kitching <kitching@chromium.org>
Reviewed-by: Joel Kitching <kitching@chromium.org>
4 files changed