Bug 18979: Speed up 'valid-templates.t' tests

Bug 18979: Speed up 'valid-templates.t' tests

Test plan:
Confirm than prove xt/author/valid-templates.t is quicker with this
patch.

Before I got:
Files=1, Tests=840, 21 wallclock secs ( 0.18 usr  0.02 sys + 20.28 cusr
0.32 csys = 20.80 CPU)

After:
Files=1, Tests=840, 17 wallclock secs ( 0.06 usr  0.01 sys + 21.56 cusr
0.48 csys = 22.11 CPU)

Signed-off-by: David Bourgault <david.bourgault@inlibro.com>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>