MDEV-5127 - Test suite test file_contents fails in Slackware Linux

Slackware install docs to different folder, which wasn't handled
by file_contents test properly. E.g. on Redhat/Debian docs are
expected to be in /usr/share/doc/mariadb-server-(version), but
on Slackware they are in /usr/doc/mariadb-(version).

For details see Slackware mariadb build script:
http://slackbuilds.org/slackbuilds/14.0/system/mariadb/mariadb.SlackBuild

mysql-test/t/file_contents.test

@@ -33,6 +33,8 @@ if ($dir_bin eq '/usr/') {
     $dir_docs = glob "$dir_docs/mariadb-server-*";
     $dir_docs = glob "$dir_docs/MySQL-server*" unless -d $dir_docs;
   }
+  # Slackware
+  $dir_docs = glob "$dir_bin/doc/mariadb-[0-9]*" unless -d $dir_docs;
 } else {
   # tar.gz package, Windows, or developer work (in BZR)
   $dir_docs = $dir_bin;