tests: provide test for vfs+fmt corner-case

On `native` when using `vfs` the `fmt` print functions do not work.
This is because the `fmt` module uses the `write()` system call which
is bend by the `native_vfs` module to use `vfs_write()`. However,
`native` does not use a `stdio` module to print. Instead, it just
writes to the hosts standard I/O directly. As such, STDIN, STDOUT, and
STDERR are never initialized for `vfs` so `vfs` does not recognize
`STDIN_FILENO`, `STDOUT_FILENO` and `STDERR_FILENO` as valid file
descriptors.

This test case showcases this bug.

tests/vfs_plus_stdio/Makefile

@@ -0,0 +1,7 @@
+DEVELHELP=0
+include ../Makefile.tests_common
+
+USEMODULE += vfs
+USEMODULE += fmt
+
+include $(RIOTBASE)/Makefile.include

tests/vfs_plus_stdio/main.c

@@ -0,0 +1,27 @@
+/*
+ * Copyright (C) 2019 Freie Universität Berlin
+ *
+ * This file is subject to the terms and conditions of the GNU Lesser
+ * General Public License v2.1. See the file LICENSE in the top level
+ * directory for more details.
+ */
+
+/**
+ * @ingroup     tests
+ * @{
+ *
+ * @file
+ * @brief       Tests VFS together with stdio on a board
+ *
+ * @author      Martine S. Lenders <m.lenders@fu-berlin.de>
+ *
+ * @}
+ */
+
+#include "fmt.h"
+
+int main(void)
+{
+    print_str("SUCCESS\n");
+    return 0;
+}

tests/vfs_plus_stdio/tests/01-run.py

@@ -0,0 +1,18 @@
+#!/usr/bin/env python3
+
+# Copyright (C) 2019 Freie Universität Berlin
+#
+# This file is subject to the terms and conditions of the GNU Lesser
+# General Public License v2.1. See the file LICENSE in the top level
+# directory for more details.
+
+import sys
+from testrunner import run
+
+
+def testfunc(child):
+    child.expect_exact('SUCCESS')
+
+
+if __name__ == "__main__":
+    sys.exit(run(testfunc, timeout=1))