Bug 21238: Prevent TemplateToolkit.t to fail on slow servers

Bug 21238: Prevent TemplateToolkit.t to fail on slow servers

It failed 3x in the last 6 runs (on node 1), we should mock
DateTime->now to prevent it to fail again.

The error is:
koha_1       |         #   Failed test at
t/db_dependent/Letters/TemplateToolkit.t line 724.

When comparing <<today>> replacement:
koha_1       |         # 08/16/2018 14:25<br />
[...]
koha_1       |         # 08/16/2018 14:24<br />

Let's mock DateTime->now to prevent it to fail again.

Test plan:
0. Do not apply this patch
1. Add "sleep(60);" at line ~715, before "# Add a second checkout"
2. Run the tests
=> Fail!
3. Apply this patch
4. Run the tests again
=> Pass!

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
(cherry picked from commit d24d77d495a56e76046907ab19e9474cab28169c)
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>