The TestShouldUseOneCGNATRoute test fails when the underlying system
interfaces don’t match what the underlying assumptions of the test.
That assumption was that there would only ever be one CGNAT interface:
the Tailscale one.
This breaks on Linux when border0 is installed because border0 also
creates an interface with a CGNAT route.
This patch stubs netmon.RegisterInterfaceGetter to replace the system
interfaces and netmon.SetTailscaleInterfaceProps to identify the test
data that defines the Tailscale interface.
This patch also tests the control knob override for CGNAT for every
combination of operating system and system interfaces, instead of just
a couple of combinations.
Fixes#19731
Signed-off-by: Simon Law <sfllaw@tailscale.com>
93dbd33ef7