tests: remove remaining exec bits ('maint' branch)

tests: remove remaining exec bits ('maint' branch)

The executable bit gives the impression that the tests are directly
runnable, as with "./t/foo.sh", but it has been a while since that
was the case.  Today, tests are runnable only through "make check"
or "./runtest".

This change is for the 'maint' branch (automake 1.13a), and is a
follow-up to commit 'v1.13.2-3-g74017b5', done on the 'micro' branch
(automake 1.13.2a).  It will soon be followed by a similar patch
for the 'master' branch (automake 1.99a).

* t/am-prog-cc-c-o.sh: Remove executable bit.
* t/ccnoco4.sh: Likewise.
* t/dist-shar.sh: Likewise.
* t/dist-tarZ.sh: Likewise.
* t/lex-multiple.sh: Likewise.
* t/preproc-basics.sh: Likewise.
* t/preproc-c-compile.sh: Likewise.
* t/preproc-demo.sh: Likewise.
* t/preproc-errmsg.sh: Likewise.
* t/rm-f-probe.sh: Likewise.
* t/self-check-cc-no-c-o.sh: Likewise.
* t/txinfo-builddir.sh: Likewise.
* t/txinfo-nodist-info.sh: Likewise.

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