tftpd: Check the server status after each test.

tftpd: Check the server status after each test.

* In the setup phase, wait for the server to start (or fail to start)
  before proceeding with the test.  This makes it possible to write test
  cases that don't expect a response from the server without ending up
  in a race over the server PID file.
* After running each test, wait up to 30 seconds for the server to exit
  and check that the exit status matches what the test case says to
  expect (usually 0).
* We still kill and collect the server in the cleanup phase, in case the
  test ended early.

MFC after:	1 week
Sponsored by:	Klara, Inc.
Reviewed by:	kevans
Differential Revision:	https://reviews.freebsd.org/D44956