tests: update PATH early from AM_TESTS_ENVIRONMENT and runtest

tests: update PATH early from AM_TESTS_ENVIRONMENT and runtest

Because the POSIX standards mandate that sourcing a file with ". FILE"
will cause FILE (assuming it is a relative containing no slashes) to be
looked for in PATH, such a move will allow us to simplify our sourcing
of shell testing libraries like (currently) 't/ax/test-init.sh' and
'defs', and to modularize and re-organize them better in the feature,
with minimal churn.

* runtest.in, Makefile.am (AM_TEST_ENVIRONMENT): Prepend $(abs_srcdir)/t/ax
and $(abs_builddir)/t/ax (in that order) to PATH; do so avoiding repetition
in the common case of non-VPATH builds.

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