selftests: Add support to nettest to run both client and server

Add option to nettest to run both client and server within a
single instance. Client forks a child process to run the server
code. A pipe is used for the server to tell the client it has
initialized and is ready or had an error. This avoid unnecessary
sleeps to handle such race when the commands are separately launched.

Signed-off-by: Seth David Schoen <schoen@loyalty.org>
Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>