Fix #447, Add rtems_sysmon unit test

[avan989]

Fix #447, Add rtems_sysmon unit test

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• I reviewed the Contributing Guide.
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Describe the contribution
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• Include explicitly what issue it addresses [e.g. Fixes #X]
  Fix Add coverage test for rtems sysmon #447, Add rtems_sysmon unit test

Testing performed
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1. Make SIMULATION=native ENABLE_UNIT_TESTS=true install

2. make test

3. make lcov

4. Run against gr712rc

Expected behavior changes
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• API Change: xxx (if applicable)
• Behavior Change: xxx (if applicable)
• Or no impact to behavior

System(s) tested on

• Hardware: [e.g. PC, SP0, MCP750]
• OS: [e.g. Ubuntu 18.04, RTEMS 4.11, VxWorks 6.9]
• Versions: [e.g. cFE 6.6, OSAL 4.2, PSP 1.3 for mcp750, any related apps or tools]

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  Anh Van