Add unsubscribe test case to cover #862

[skliper]

Is your feature request related to a problem? Please describe.
#862 looks like it could have resulted in a seg fault, fixed in #865
Unsubscribe after no subscriptions are left...

Describe the solution you'd like
Add test case.

Describe alternatives you've considered
None. Should add test to confirm code works.

Additional context
#862 #865

Requester Info
Jacob Hageman - NASA/GSFC
@CDKnightNASA

[CDKnightNASA]

Looks like current tests hits it ... lcov report shows that lines 1102-1110 were covered one time. But I will assume you mean that you'd like an explicit test.

[skliper]

Note `Test_Unsubscribe_NoMatch) doesn't actually unsubscribe from MsgId or SB_UT_TLM_MID1, so it's not checking the unsubscribe followed by unsubscribe case. The test modifies the PipeId for the MsgId description such that it won't unsubscribe. I had to dig a bit since the test is light on comments as to the "why"...

cFE/fsw/cfe-core/unit-test/sb_UT.c

Lines 2520 to 2544 in be1a23a

	void Test_Unsubscribe_NoMatch(void)
	{
	CFE_SB_PipeId_t TestPipe;
	CFE_SB_MsgId_t MsgId = SB_UT_TLM_MID;
	CFE_SB_MsgRouteIdx_t Idx;
	uint16 PipeDepth = 50;
	SETUP(CFE_SB_CreatePipe(&TestPipe, PipeDepth, "TestPipe"));
	SETUP(CFE_SB_Subscribe(MsgId, TestPipe));
	ASSERT(CFE_SB_Unsubscribe(SB_UT_TLM_MID1, TestPipe));
	/* Get index into routing table */
	Idx = CFE_SB_GetRoutingTblIdx(CFE_SB_ConvertMsgIdtoMsgKey(MsgId));
	CFE_SB.RoutingTbl[CFE_SB_RouteIdxToValue(Idx)].ListHeadPtr->PipeId = 1;
	CFE_SB.RoutingTbl[CFE_SB_RouteIdxToValue(Idx)].ListHeadPtr->Next = NULL;
	ASSERT(CFE_SB_Unsubscribe(MsgId, TestPipe));
	EVTCNT(7);
	EVTSENT(CFE_SB_UNSUB_NO_SUBS_EID);
	TEARDOWN(CFE_SB_DeletePipe(TestPipe));
	} /* end Test_Unsubscribe_NoMatch */