test: add a --post-start hook

This hook runs after the test is started, in parallel with the actual test.
This can be used in case the test and the host need to coordinate somehow
in order to set something up (e.g. for a macvlan interface).

travis-ci: success for series starting with [v10,01/11] net: pass the struct nlattrs to dump() functions
Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>

test/zdtm.py

@@ -374,6 +374,8 @@ class zdtm_test:
 
 		s = subprocess.Popen(s_args, env = env, cwd = root, close_fds = True,
 				preexec_fn = self.__freezer and self.__freezer.attach or None)
+		if act == "pid":
+			try_run_hook(self, ["--post-start"])
 		s.wait()
 
 		if self.__freezer: