Minor clean up for refdata.*

Minor clean up for refdata.*

- Remove global functions that are no longer necessary.  All the state
  about the reference data mode is stored internally in refdata.cpp.
  These were remnants from an old implementation where the code was
  split into multiple .cpp files to support compilation of testutils/
  without libxml2 dependencies.
- Make the self-tests for the reference data code not use the source
  tree as a location for the transient data they create.

In addition to clarifying the code, these should make it harder to
misuse the constructor intended for self-testing, since it will be
difficult to pass Jenkins if someone tries to use it for other tests.

Change-Id: I204ddadfc224a79c587f0588a4d510b7b8486f9b