tests: new variable $am_testaux_builddir

tests: new variable $am_testaux_builddir

And a related fix that solves a spurious testsuite failures in
VPATH builds.

* t/ax/test-defs.in ($am_testaux_builddir): New, counterpart of
$am_testaux_srcdir, but pointing inside the build directory.
* t/self-check-shell-no-trail-bslash.sh: Use $am_testaux_builddir,
not $am_testaux_srcdir, when fetching the 'shell-no-trail-bslash'
script.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>