tests: simplify sourcing of helper shell files

tests: simplify sourcing of helper shell files

Now that the early $PATH setup in both 'runtest' and AM_TESTS_ENVIRONMENT
allow the '.' built-in to find to-be-sourced shell scripts in the 't/ax/'
directory automatically, we can simplify several usages like:

   . "$am_testauxdir"/foo.sh

to just:

   . foo.sh

Also, because our test scripts run with the 'errexit' flag active, and
because POSIX mandates that, when the '.' built-in is used,

   ... if no readable file is found, a non-interactive shell shall abort ...

we can further simplify usages like:

    . "$am_testauxdir"/foo.sh || fatal_ "sourcing foo.sh"

once again to to just:

    . foo.sh

* ./defs, several tests: Adjusted.

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