Bug 16174: Warning in tt_valid.t due to bug 15527

 Bug 16174: Warning in tt_valid.t due to bug 15527

Bug 15527 introduced a new directory (xslt) on the top level OPAC templates
directory. The tt_valid.t script should include that top level dir on the list
of excluded dirs so it doesn't look for themes there.

To test:
- Run
  $ prove xt/tt_valid.t
=> FAIL: You get the following warning:
Can't stat koha-tmpl/opac-tmpl/xslt/en: No such file or directory
 at xt/tt_valid.t line 8
- Apply the patch
- Run
  $ prove xt/tt_valid.t
=> SUCCESS: Tests pass and don't raise warnings.

Signed-off-by: Mark Tompsett <mtompset@hotmail.com>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Brendan A Gallagher <brendan@bywatersolutions.com>