Tests for proxy interface handling

[bahusoid]

Related to #2052 and test case for #2271

Added tests to clarify expected behavior.
In 5.2 both implicit and explicit interfaces are not leading to proxy initialization on interface Id access.

Update: After #2264 implicit interfaces are not leading to proxy initialization on Id access. And explicit interfaces lead to proxy initialization. Tests are adjusted according to this behavior.

[bahusoid]

I've updated tests according to changes made in #2264. Also added test case for #2271

[bahusoid]

It does test that an explicitly implemented member is not "short-circuit" to its implicitly implemented "counter-part".

I wrote my tests a while ago - see no point dropping them now. Various scenarios for interface handling in one place. Also your tests seem to check some internal specifics of generated proxy. And my tests make no assumptions what should cause proxy initialization - they just test actual initialization. So they more abstract from actual implementation.

Regarding #2271. Maybe it's just invalid test case and issue - will double check tomorrow. Double checked - it's valid issue.

[hazzik]

Checked locally that there is no changes in async code.