tests: rename $am_testauxdir -> $am_testaux_srcdir

tests: rename $am_testauxdir -> $am_testaux_srcdir

This is just a preparatory change in view of a future commit.

* t/ax/test-defs.in: Here.
* t/ax/tap-summary-aux.sh: And here.
* t/ax/testsuite-summary-checks.sh: And here.
* t/distcheck-missing-m4.sh: And here.
* t/distcheck-outdated-m4.sh: And here.
* t/self-check-shell-no-trail-bslash.sh: And here.
* t/test-driver-acsubst.sh: And here.
* t/test-driver-cond.sh: And here.
* t/test-driver-custom-multitest.sh: And here.
* t/test-driver-custom-multitest-recheck.sh: And here.
* t/test-driver-custom-multitest-recheck2.sh: And here.
* t/testsuite-summary-count-many.sh: And here.

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