tests: decouple network perf test from pin_threads layout

The network throughput test recomputed the first free CPU as
`vcpus_count + 2`, duplicating pin_threads' internal thread layout.
Move pin_threads() out of the network_microvm fixture into the tests so
test_network_tcp_throughput can consume pin_threads' documented return
value instead, and assert against it in the iperf helper.

Signed-off-by: James Curtis <jxcurtis@amazon.co.uk>