Fix build warnings and unit test improvements

[muneebahmed10]

Description:
Several calls to expectProcessLoopCalls in core_mqtt_utest.c used the MQTTStateNull enum instead of MQTTSuccess, and resulted in compiler warnings from llvm:

 ◌ libraries/standard/coreMQTT/test/unit-test/core_mqtt_utest.c:
     1693:39  	clang       	warning   	Implicit conversion from enumeration type 'enum MQTTPublishState' to different enumeration type 'MQTTStatus_t' (aka 'enum MQTTStatus')
     1704:39  	clang       	warning   	Implicit conversion from enumeration type 'enum MQTTPublishState' to different enumeration type 'MQTTStatus_t' (aka 'enum MQTTStatus')
     1715:39  	clang       	warning   	Implicit conversion from enumeration type 'enum MQTTPublishState' to different enumeration type 'MQTTStatus_t' (aka 'enum MQTTStatus')
     1725:39  	clang       	warning   	Implicit conversion from enumeration type 'enum MQTTPublishState' to different enumeration type 'MQTTStatus_t' (aka 'enum MQTTStatus')
     1754:39  	clang       	warning   	Implicit conversion from enumeration type 'enum MQTTPublishState' to different enumeration type 'MQTTStatus_t' (aka 'enum MQTTStatus')

Since expectProcessLoopCalls has many parameters, it's not obvious from looking at it in each call what each parameter is doing, and whether it's actually necessary. This introduces a structure to hold expectProcessLoopCalls' parameters, and only the parameters that are actually used will have to be changed in each invocation.