qa/run_xfstests.sh: quit building xfstests on test nodes

qa/run_xfstests.sh: quit building xfstests on test nodes

xfstests is a pain to build on trusty, xenial and centos7 with a single
script.  It is also very sensitive to dependencies, which again need to
be managed on all those distros -- different sets of supported commands
and switches, some versions have known bugs, etc.

Download a pre-built, statically linked tarball and use it instead.
The tarball was generated using xfstests-bld by Ted Ts'o, with a number
of tweaks by myself (mostly concerning the build environment).

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>