net: tests: Use lighter printk() instead of printf()

Use printk(), snprintk() instead of printf() and snprintf().
CONFIG_STDOUT_CONSOLE is anyway disabled by default so printf()
will not output anything without it.

Change-Id: I5899741df2e187dae7b4602d074e9b4cbb9952aa
Signed-off-by: Ravi kumar Veeramally <ravikumar.veeramally@linux.intel.com>