Bug 25018: Use new KOHA_TESTING envvar to detect environment testing

Bug 25018: Use new KOHA_TESTING envvar to detect environment testing

In some tests we want to know if we are in a testing environment.
When run the usual way, our trick works, the perl interpreter matches 'prove':
  $ENV{_} eq 'prove'

In other situations, we have the KOHA_NO_TABLE_LOCKS environment variables, for the SendCirculationAlert race conditions (see bug 15854 and bug 18364).

For unknown reasons, Jenkins runs the tests with /usr/bin/perl.

This patch suggests to rename KOHA_NO_TABLE_LOCKS and use KOHA_TESTING
instead, when prove is not used (or not correctly detected as it it the
case for Jenkins)

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>