Integrate Code Coverage feature

[Earewien]

To pass from alpha to beta, it would be nice to offer a nice functionnality.

When running unit tests, there is a Holy Trinity : Run, Debug and Coverage. Run and Debug are already implemented, but not Coverage.

How to do it ?

Naively, in 3 steps :

• Create a GDScript AST. It will be reused by ParsedGDScript later to parse and extract values from GDScripts
• From AST, when running a test case, transform each script into a instrumented script. Instrumented GDScript contains instructions to calculate coverage. Those instructions are side effects that communicate to runner a unique identifier that identifies the covered instruction.
• Show coverage report in script editor