C#: Add tests for extractor fixes, and improve CFG for ConstCases

[calumgrant]

These tests will not pass until the corresponding extractor changes have been merged.

Move the getCondition() predicate from ConstCase to CaseStmt, because const cases can also have when parts, that were not previously extracted.

Implement the CFG for ConstCases that have a when part.

Add some regression tests for extractor fixes.

[hvitved]

Very nice! Just a few remarks.

[hvitved]

Two more comments, but nothing urgent, so happy to merge as is.

[hvitved]

I am sure I wrote this originally, but github seems to have thrown my comment away:

I would replace

          // Flow from last element of case expression to first element of statement
          not exists(cc.getCondition()) and
          cfe = lastConstCaseExpr(cc, c) and
          c.(MatchingCompletion).isMatch() and
          result = first(cc.getStmt())
          or
          // Flow from the last element of case expression to the condition
          cfe = lastConstCaseExpr(cc, c) and
          c.(MatchingCompletion).isMatch() and
          result = first(cc.getCondition())

with

          cfe = lastConstCaseExpr(cc, c) and
          c.(MatchingCompletion).isMatch() and
          if exists(cc.getCondition()) then
            // Flow from the last element of case expression to the condition
            result = first(cc.getCondition())
          else
	    // Flow from last element of case expression to first element of statement
            result = first(cc.getStmt())

[hvitved]

Could replace with

result = last(cs.(LabeledStmt).getStmt(), c)

Feel free to change if you like.

[calumgrant]

I did that initially but additional edges appeared.