Extend the `axi_intf.sv` file with significant test functionality. The
`axi_test` package now provides a `axi_driver` and `axi_lite_driver`
that can be used to send and receive transactions on each of the AXI and
AXI-Lite channels.

Add a behavioural testbench for the `axi_lite_to_axi` adapter. It does
not properly check anything yet, but rather triggers a single AXI
transaction.

Add a synthesis testbench that instantiates the adapters with the
different parameters supported. All data widths are tested, and a few
select ID, user, and address widths.

Hook everything up into GitLab CI.