Add unit tests that increase code coverage (Go/Golang/SenML)

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A "good first issue" with two programs at 0% coverage (inherited from cisco/senml).

Code coverage using go test -cover shows:

• 80.7% for senml.go
• 0% for senmlCat.go
• 0% for senmlServer.go

Add unit tests that improve code coverage. If possible, use example data directly from standards docs like RFC 8428 or gathered from sensors (as long as data doesn't violate privacy or laws).

Hint: Tests involving io.Reader or io.Writer can use iotest to increase coverage after the normal path is covered.

Before submitting a pull request, please reply here with your proposed changes.