netinet tests: Make some tests more reliable when run in parallel

Many of the modified tests add epair interfaces to the host, though most
of the tests themselves run in a VNET jail. scapy in particular becomes
unhappy when the list of interfaces changes as it is running, so, to
improve reliability of parallel test runs, isolate tests which add
epairs to the host.

Also serialize arp tests, as they examine the dmesg as part of the test.

The list of modified tests is not exhaustive, it was determined by
running the test suite with parallelization enabled and looking at
failures. It may be easier to just automatically enable VNET jailing of
all netinet tests, but let's be more particular for now.

Reviewed by: kp
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D46606