tests: test VNets functionality as a blackbox

tests: test VNets functionality as a blackbox

Add several tests for Vnets in test_vnets_blackbox. State setup as well
as testing results is done only via the API to test on the API
boundaries not not against the internal state. Internal state is mocked
to avoid requiring access to system files or pmxcfs.

Mocking is done by reading and writing to a hash that holds the entire
state of SDN. The state is reset after every test run.

Testing is done via helper functions: nic_join and nic_start.
When a nic joins a Vnet, currently it always - and only - calls
add_next_free_cidr(). The same is true if a nic starts on Vnet, which
only calles add_dhcp_mapping.

These test functions homogenize the parameter list in contrast to the
current calls to the current functions.  The intention for the functions
is that they can be moved to Vnets.pm to be called from QemuServer and
LXC!

The tests are composed of a test function which can be parameterized. To
call the test function, the run_test function takes the function pointer
and passes the rest of the arguments to the test functions. It also
takes care of resetting the test state.
This allows fine-grained parameterization per-test directly in the code
instead of separated files that require the entire state to be passed
in.

The tests setup the SDN by creating a simple zone and a simple vnet. The
nic_join and nic_start function is called with different subnet
configuration wiht and without a dhcp-range configured and with or
without an already present IP in the IPAM.

Signed-off-by: Stefan Lendl <s.lendl@proxmox.com>
Reviewed-by: Max Carrara <m.carrara@proxmox.com>
Tested-by: Max Carrara <m.carrara@proxmox.com>
Reviewed-by: Stefan Hanreich <s.hanreich@proxmox.com>
Tested-by: Stefan Hanreich <s.hanreich@proxmox.com>