Bug 14957: (QA follow-up) Clarify 'context' param

Bug 14957: (QA follow-up) Clarify 'context' param

This patch renames the (passed through) 'context' param for
'overlay_context'. I propose doing so, because in Koha-land 'context'
has a special meaning, related to C4::Context and it reads ambigous.

The patch itself is pretty trivial.

Tests should pass:
1. Run:
   $ kshell
  k$ prove t/db_dependent/Biblio/MarcOverlayRules.t
=> SUCCESS: Tests pass
2. Apply this patch
3. Repeat 1
=> SUCCESS: Tests still pass!
4. Sign off :-D

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Bug 14957: (follow-up) Clarify 'context' param

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