Bug 24494: tests: 00-valid-xml.t shouldn't check node_modules

Bug 24494: tests: 00-valid-xml.t shouldn't check node_modules

== Test plan ==
- work with SCSS (just recompile it if you never did)
  - https://wiki.koha-community.org/wiki/Working_with_staff_client_SCSS
- now you have a node_modules directory
- prove -r t/00-valid-xml.t
- it should fail on files in node_modules
- apply this patch
- prove -r t/00-valid-xml.t
- it should work
- sabotage a legitimate XML file, like etc/SIPconfig.xml
- prove -r t/00-valid-xml.t
- it should fail, good the test is still useful
- undo the sabotage

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>