Add unit tests for specific tag attribute suggestions and closing tag completions

[nlewis05]

This will need to be part of the testing update initiative in 1.4.0(#789), testing whether this can be done is based on what tool we decide to use. Ex: Go to a specific line of code, use ctrl + space, and test the result

[rthomas320]

I was asked to comment on a possible solution to this issue. What we need is the ability to "play" (mimic?) the actions described in src/test/DfdlIntellisenseTestingChecklist.md and test the results in a unit test. I not sure if there are any current tools that allow that functionality.