pf tests: support packet size range in pft_ether.py

Teach pft_ether.py to send a range of packet sizes. Use this to move the
size sweep into Python, removing the repeated Python startup overhead
and greatly speeding up the pf.ether.short_pkt test.

This should fix test timeouts seen on ci.freebsd.org.

While here also extend the range of packet sizes tested, because it adds
very little runtime now.

Sponsored by: Rubicon Communications, LLC ("Netgate")

pf tests: test short packets

Test sending very short packets (i.e. too short for an IP header)
packets in the Ethernet filtering code.

Sponsored by: Rubicon Communications, LLC ("Netgate")