We currently assume there are two links between the SUT and TG nodes,
but that's too strict, even for some of the already existing test cases.
Add support for topologies with less than two links.

For topologies with no links, dummy ports are used. The expectation is
that test suites or cases that don't require any links won't be using
methods that use ports. Any test suites or cases requiring links will be
skipped in topologies with no links, but this feature is not implemented
in this commit.

Signed-off-by: Juraj Linkeš <juraj.linkes@pantheon.tech>