It used to be possible to run test transactions against mercurypay.com
using a regular swiper device, test credentials and test cards
provided by Mercury.

Mercury has recently disabled this saying this feature did not adhere
to certain PCI regulations.

It means that currently on the regular mercurypay.com network only
real cards (and thus real money) can be used.

Because we require a way to test without spending money this commit
allows to switch to the test network (mercurycert.net). It uses a
system parameter because we cannot introduce a Boolean on
pos_mercury.configuration in a stable release. It also changes the
test credentials to some that work on mercurycert.net.

Note that the regular 'production' swiper devices can not be used on
this test environment. This results in:

Error 004118: Failed to decrypt

The swiper contains embedded cryptographic keys specific to
mercury*pay*.net. Instead a specific 'development' swiper device with
embedded keys for mercurycert.net is required.

opw-1832156 (this is necessary to test that issue)