tests: net: lib: coap: Added new functions to improve code coverage
tests: net: lib: coap: Added new functions to improve code coverage

In coap module previous test cases are modified as well as
new test functions are added to increase the code coverage

Signed-off-by: Tarun Kumar Agarwal <fd8a70af5d7ddae34d7c279461d41fc015167f9f@intel.com>

finikorg/zephyr,GiulianoFranchetto/zephyr,zephyrproject-rtos/zephyr,kraj/zephyr,finikorg/zephyr,nashif/zephyr,galak/zephyr,nashif/zephyr,ldts/zephyr,Vudentz/zephyr,punitvara/zephyr,zephyrproject-rtos/zephyr,GiulianoFranchetto/zephyr,nashif/zephyr,ldts/zephyr,Vudentz/zephyr,finikorg/zephyr,ldts/zephyr,explora26/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,explora26/zephyr,ldts/zephyr,ldts/zephyr,explora26/zephyr,punitvara/zephyr,galak/zephyr,galak/zephyr,Vudentz/zephyr,kraj/zephyr,explora26/zephyr,Vudentz/zephyr,GiulianoFranchetto/zephyr,zephyrproject-rtos/zephyr,punitvara/zephyr,GiulianoFranchetto/zephyr,punitvara/zephyr,kraj/zephyr,finikorg/zephyr,GiulianoFranchetto/zephyr,kraj/zephyr,galak/zephyr,Vudentz/zephyr,galak/zephyr,Vudentz/zephyr,nashif/zephyr,kraj/zephyr,explora26/zephyr,finikorg/zephyr,punitvara/zephyr,zephyrproject-rtos/zephyr