In test mode, dynamically load libraries in well known locations that can
allow to override the libfprint behavior.
It would still be possible to potentially inject code by replicating the
distro build directory and adding a library there, but if one is able to
access there, they would already be able to access any path.
Plus the env variable check is still there, so again they would need to
be able to change the fprintd environment
Rather than just do it in production if set, make it more difficult to
potentially inject the test mode.
So rely on a weak definition of fpi_device_emulation_mode_enabled() that
can be only set externally
