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

[Karl Berry]

    When programs are run
    during the build, files (and texmf.cnf) should be found relative to
    the binaries in the Build tree, regardless of what else is in PATH.  

The basic point of my message was that that assumption is incorrect.  I
agree in theory that it "should" be this way, but there are nontrivial
issues with making it be so.  As I explained.

The relevant thing to try to figure out is not the test infrastructure,
but whether the pmpost crash still happens and if so, why.  -k