net: apps: Add sanity test changes to test_15_4

Add sanity test related changes to test_15_4 application in both
micro and nano kernel. config_whitelist in testcase.ini is not
accepting other than PLATFORM configs. So creating test folder
for sanity tests. src directory is still common for all.

Change-Id: Iffa4c588500367fa475aef8e0772f1c08946c5a4
Signed-off-by: Ravi kumar Veeramally <ravikumar.veeramally@linux.intel.com>