logging/vty: fix: actually ignore deprecated logging commands

We shall not prevent programs from starting if their configuration
files contain deprecated 'logging level ...' commands. Just print
a warning and return CMD_SUCCESS instead of CMD_WARNING.

While writing a unit test, another funny bug has been uncovered.
Parsing of a deprecated command indeed triggers a deprecation
warning, originated from libosmovty's log_deprecated_func().
This function simply calls vty_out(), but...

Since the invocation of the vty_out() happens _before_ the VTY
is initialized, the process is actually writing that warning
to its own stdin! Most likely, because we use talloc_zero()
to allocate a new instance of struct 'vty'.

As a side effect, the evil warning magically appears in the output
of 'make check', breaking the test statistics. Let's work around
this bug for now by redirecting stdin to /dev/null.

Change-Id: Ia934581410cd41594791d4e14ee74c16abe1009a
Fixes: Ic9c1b566ec4a459f03e6319cf369691903cf9d00
diff --git a/tests/vty/ok_deprecated_logging.cfg b/tests/vty/ok_deprecated_logging.cfg
new file mode 100644
index 0000000..2699719
--- /dev/null
+++ b/tests/vty/ok_deprecated_logging.cfg
@@ -0,0 +1,3 @@
+log stderr
+ logging filter all 1
+ logging level depr debug