test: verify used variables in different scenarios

[nikku]

Proposed Changes

Validate scenarios for used variables, discussed here.

Scenario 1: Pro-code setup

This adds test cases for used variables - in the extreme case where no variables are produced ("pro-code"):

Scenario 2: Variable written and read (#91)

Validate representation of variable that is read and written by a task:

Scenario 3: Variable attaches to correct scope (#92)

Verify that usedBy meta-data is attached to relevant scope:

Scenario 4: Proposed filtering mechanism (#93)

Verify the newly proposed filtering mechanism works:

From the documentation:

// variables available in scope of <task>
await variableResolver.getVariablesForElement(task);

// variables read by <task>, excluding local ones
await variableResolver.getVariablesForElement(task, { read: true, local: false });

// all variables written by <task>
await variableResolver.getVariablesForElement(task, { written: true });

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[nikku]

Incorporated all tests into this PR - including new tests for filtering behavior - #93.