[tlbuild] test suite fails when TL2015 binaries are in PATH

[Karl Berry]

Well, clearly the tests should not depend on any such ordering.  But the
even bigger problem is that you are seeing a core dump evidently while
reading plain.mp.  Sigh.  I don't even know who is responsible for
"upMetaPost".  Akira, Norbert, can one of you help?
(No, I do not see this crash.)  Thanks, K.

This is upMetaPost, version 1.9991-0.04-u1.21 (TeX Live 2016) (kpathsea version 6.2.2) (utf8.uptex)
(./upmpsamp.mp (/xopt/texlive/2015/texmf-dist/metapost/base/plain.mpupmpost: ./pmp.w:5641: do_set_attr_head: Assertion `A->type==mp_structured' failed.

Preloading the plain mem file, version 1.005../../../texk/web2c/pmpostdir/upmpsamp.test: line 38:  8855 Aborted                 (core dumped) ./upmpost -ini -tex=euptex upmpsamp