Add a pending test that shows a problem with instability of representing
self variables. This test covers the bug described in #2504.

In order to test API representation of a class declared either in source
file or unpickled from a class file, ScalaCompilerForUnitTesting has been
extended to extract APIs from multiple compiler instances sharing a
classpath.

The reason for instability is a bit tricky so let's unpack what the
previous code checking if there's self type declared was doing. It would
check if `thisSym` of a class is equal to a symbol representing the class.
If that's true, we know that there's no self type. If it's false, then
`thisSym` represents either a self type or a self variable. The second
(type test) was supposed to check whether the type of `thisSym` is
different from a type of the class. However, it would always yield false
because TypeRef of `thisSym` was compared to ClassInfoType of a class.
So if you had a self variable the logic would see a self type (and that's
what API representation would give you).

Now the tricky bit: `thisSym` is not pickled when it's representing just
a self variable because self variable doesn't affect other classes
referring to a class. If you looked at a type after unpickling, the
symbol equality test would yield true and we would not see self type when
just a self variable was declared.

The fix is to check equality of type refs on both side of the type equality
check. This makes the pending test passing.

Also, I added another test that checks if self types are represented in
various combinations of declaring a self variable or/and self type.

Fixes #2504.