tests: prefer including 'test-init.sh' rather than './defs'

tests: prefer including 'test-init.sh' rather than './defs'

This is a follow-up to today's commit v1.12.4-22-g0610fc8,
"tests: prepare to move ./defs to t/ax/test-init.sh"

* All tests: To run the common setup, use the command:

    . test-init.sh

instead of the older, "historical" one:

    . ./defs || exit 1

Note that the "|| exit 1" wasn't really useful, since the 'errexit'
shell flag is in effect in both './defs' and 'test-init.sh', and all
the known shells that are good enough to run the automake testsuite
do automatically exit with error when a sourced file cannot be found
(at least, they do so in non-interactive mode, which is the only
mode that concerns us in the testsuite).

* t/ax/tap-summary-aux.sh, t/ax/testsuite-summary-checks.sh: Likewise.

* gen-testsuite-part: Do the same in the generated tests.

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