Check that libxml is installed before building
Don't use the job id in its description

it has no useful meaning outside the library and it creates spurious
cascading differences against golden output when one little thing goes
awry.
Reimplement tnparallel post script in Perl

and as a normalisation filter rather than a post filter.

Also, make it explicitly reorder the PASS statements it sees so that
they are predictable.  We really don't care about the order, only that
each test is run and passed.