htmlxref htmlxref-only_mono htmlxref-only_split tests

[Gavin Smith]

I think it would be simpler to do all the tests under tp/tests in a
single directory, instead of using the three subdirectories htmlxref,
htmlxref-only_mono and htmlxref-only_split. For example, a test
failure in the upper directory can stop the tests in the lower
directories being run. I feel it may make it more complicated to
update test results as well. Before I spend too much time trying to
work out how to run the tests in these subdirectories like the other
tests under tp/tests, I thought I'd mention it here in case there was
some reason why it would be awkward to do.