Bug 18055: Speed up '00-strict.t' test, with Parallel::ForkManager

Bug 18055: Speed up '00-strict.t' test, with Parallel::ForkManager

add KOHA_PROVE_CPUS envar to manually set number of cpus

to test..

1/ set number of cpus to 1
 $ export KOHA_PROVE_CPUS=1

2/ observe 1 cpus are used
 $ time prove -v  t/db_dependent/00-strict.t
 t/db_dependent/00-strict.t ..
 Using 1 CPUs...

3/ unset number of cpus
 $ export KOHA_PROVE_CPUS=

4/ observe all available cpus are used
 $ time prove -v  t/db_dependent/00-strict.t
 t/db_dependent/00-strict.t ..
 Using 32 CPUs...

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>