Need appropriate method(s) to fabricate valid MsgIds

[jphickey]

Is your feature request related to a problem? Please describe.
In certain circumstances - particularly the SB functional tests - there is a need to subscribe to a large quantity of MsgIds. These don't need to be MsgIds on which data is actually published - in fact its preferable if its not.

Previously, the testcase code used the BASE msgid define, but this wasn't guaranteed to be there (user config dependent) so it was changed to simply 1 + NumSubs. However, this makes a different assumption that anything nonzero will pass the CFE_SB_IsValidMsgId() test.

Describe the solution you'd like
This should use a more robust method to generate MsgIds that are structurally valid (in terms of the bits/bit patterns) but not actually carrying live traffic.

Additional context
SB subscription test is here:

cFE/modules/cfe_testcase/src/sb_subscription_test.c

Lines 208 to 215 in 1d6a761

	/* fabricate a msgid to subscribe to (this may overlap real msgids) */
	TestMsgId = CFE_SB_ValueToMsgId(1 + NumSubs);
	Status = CFE_SB_Subscribe(TestMsgId, PipeId);
	if (Status != CFE_SUCCESS)
	{
	break;
	}

Requester Info
Joseph Hickey, Vantage Systems, Inc.