test: create .netdev file at last

test: create .netdev file at last

Previously, when the test ran on mkosi, then networkd was not masked, and
might be already started. In that case, the interface test2 would be created
soon after the .netdev file is created, and the .link file would not be
applied to the interface. Hence, the later test case for
'networkctl cat @test2:link' would fail.

This make networkd always started at the beginning of the test, and
.netdev file created after .link file is created. So, .link file is
always applied to the interface created by the .netdev file.