tests: make script 'shell-no-trail-bslash' simpler and more robust

tests: make script 'shell-no-trail-bslash' simpler and more robust

This solves spurious failure in the 'check-no-trailing-backslash-in-recipes'
target for Automake-NG.

This is basically a backport of Automake-NG commit v1.14.1-1010-g85aae58;
the point is to minimize the amount of spurious diffs between the mainline
Automake and the Automake-NG source trees.

* t/ax/shell-no-trail-bslash.in: Simplify and fortify.
* t/self-check-shell-no-trail-bslash.sh: Enhance.

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