tests: merge two tests on silent-rules with libtool

tests: merge two tests on silent-rules with libtool

This helps to reduce code duplication, and also to (marginally)
speed up the involved tests.

Before this change, we had:

  $ diff -u t/silent3.sh t/silent9.sh

  --- t/silent3.sh      2012-11-07 19:38:05.000000000 +0100
  +++ t/silent9.sh      2012-11-07 19:27:54.000000000 +0100
  @@ -14,9 +14,9 @@
   # You should have received a copy of the GNU General Public License
   # along with this program.  If not, see <http://www.gnu.org/licenses/>.

  -# Check silent-rules mode, with libtool, standard depmode case.
  +# Check silent-rules mode, with libtool, nodep case.

  -# Please keep this file in sync with 'silent4.sh' and 'silent9.sh'.
  +# Please keep this file in sync with 'silent3.sh' and 'silent4.sh'.

   required='cc libtoolize'
   . test-init.sh
  @@ -26,8 +26,8 @@
   cat >>configure.ac <<'EOF'
   AC_CONFIG_FILES([sub/Makefile])
   AC_PROG_CC
  -AM_PROG_AR
   AM_PROG_CC_C_O
  +AM_PROG_AR
   AC_PROG_LIBTOOL
   AC_OUTPUT
   EOF
  @@ -61,7 +61,7 @@
   $AUTOMAKE --add-missing
   $AUTOCONF

  -./configure --enable-silent-rules
  +./configure --disable-dependency-tracking --enable-silent-rules
   $MAKE >stdout || { cat stdout; exit 1; }
   cat stdout
   $EGREP ' (-c|-o)' stdout && exit 1

* t/silent9.sh: Remove, merge ...
* t/silent3.sh: ... in here.
* t/list-of-tests.mk: Adjust.

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