zfs-tests: fix warnings when packaging some .shlib files

This change prevents the following warning when packaging some zfs-tests
files:

   *** WARNING: ./usr/src/zfs-0.8.0/tests/zfs-tests/include/zpool_script.shlib
   is executable but has empty or no shebang, removing executable bit

Reviewed by: John Kennedy <john.kennedy@delphix.com>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Giuseppe Di Natale <guss80@gmail.com>
Signed-off-by: loli10K <ezomori.nozomu@gmail.com>
Closes #8787

Makefile.am

@@ -111,9 +111,10 @@ mancheck:
 	fi
 
 testscheck:
-	@find ${top_srcdir}/tests/zfs-tests/tests -type f \
+	@find ${top_srcdir}/tests/zfs-tests -type f \
 		\( -name '*.ksh' -not -executable \) -o \
 		\( -name '*.kshlib' -executable \) -o \
+		\( -name '*.shlib' -executable \) -o \
 		\( -name '*.cfg' -executable \) | \
 		xargs -r stat -c '%A %n' | \
 		awk '{c++; print} END {if(c>0) exit 1}'