Bug 20181: (follow-up) Ensure test runs

Bug 20181: (follow-up) Ensure test runs

If EnablePlugins was 0, this test before this patch fails.
After this patch, it passes.

TEST PLAN
---------
1) Apply all but this patch
2) Run the following commands
    kshell
    prove t/Koha_Template_Plugin_KohaPlugins.t
   -- two tests fail, if you have the conf or syspref set to 0.
3) Apply this patch
4) Repeat step 2.
   -- all tests pass, including the positive and negative case checks.
5) run koha qa test tools

Signed-off-by: Mark Tompsett <mtompset@hotmail.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>