Start writing up some basic feature tests for procstat

These tests query a running process for information related to the -b,
-c, -e, and -f flags; the -f testcase is largely stubbed out, pending
additional work to determine a good, deterministic descriptor.

Core file test support is coming soon--it requires a bit more effort
due to the fact that:

• coredumps can be disabled (kern.coredump=0).
• corefiles can be put in different directories than the current directory, or be named something other than <prog>.core (kern.corefile).

MFC after: 2 months
Sponsored by: Dell EMC Isilon