CFE testcase code should define its messages in proper header file

[jphickey]

Is your feature request related to a problem? Please describe.
The "cfe_testcase" module has its own messages and msgids that it uses to test SB. However these are mostly just defined within the C file that has the test source code.

Describe the solution you'd like
This should follow the proper methods established elsewhere and define these messages and msg IDs in a separate header, using the current design and naming patterns.

Additional context
It may be just test code but there is no good reason to ignore the proper methods for doing this type of thing.

Requester Info
Joseph Hickey, Vantage Systems, Inc.