`testcase-extractor`: Support for `decision>context>contextEntry>literalExpression`

`testcase-extractor` does not support  `decision>context>contextEntry>literalExpression` in DMN so these test case are exported in wrong state. See e.g. test for 
```
dir: /TestCases/compliance-level-3/1130-feel-interval
model: 1130-feel-interval.dmn
```

Related PR is #21 