test(AstCheckerTest): Refactor isSurcharged

[slarse]

#4100

This is a refactor of AstCheckerTest#isSurcharged(). It should be noted that I don't understand what "surcharged" means in the context, and I also don't understand the intent of the function. The mechanics are however clear to me, see the invariants in 32de37d.

An invocation v is a potential surcharge delegate if it:

• Is an invocation that is the only statement in the body.
• Is an invocation that is the first statement in the body, and the last statement is a return without an invocation.
• Is an invocation in a return occurring as the last statement in the body.

v is determined to be a surcharge delegate if it fulfills any of the 3 above points, and returns false for isToBeProcessed(v).

[slarse]

I was going to keep going here but I think this refactoring will be sufficient.

Ping @monperrus for review, I need to get on with the rest of this class so that I unblock @I-Al-Istannen's PR #4038 which has been blocked for way too long.

[monperrus]

ack, thanks for the test improvement!