test: add tests for split std{err,out}

Now that we have support for split std{out,err}, make sure that execsync
will correctly handle the split stdio properly. In addition, extend the
ctr logging test to make sure that the regular container logging is also
split correctly. We can't test !terminal containers because we only have
a single console for both std{out,err}.

Signed-off-by: Aleksa Sarai <asarai@suse.de>

test/testdata/container_config_logging.json

@@ -7,10 +7,10 @@
 		"image": "docker://busybox:latest"
 	},
 	"command": [
-		"/bin/echo"
+		"/bin/sh", "-c"
 	],
 	"args": [
-		"%echooutput%"
+		"%shellcommand%"
 	],
 	"working_dir": "/",
 	"envs": [