tests: avoid a "grep -E" portability issue

tests: avoid a "grep -E" portability issue

As stated in the Autoconf manual, the empty alternative is not portable
with "grep -E".  For example, while with GNU grep we have:

    $ echo b | grep -E '(|a)b'; echo status = $?
    b
    status = 0

with Solaris 10 /usr/xpg4/bin/grep we have:

    $ echo b | grep -E '(|a)b'; echo status = $?
    1
    status = 1

* t/silent-texi.sh: That was causing a spurious failure here.  Fix it
by simply using '?' instead (as suggested by the Autoconf manual).

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