changed RETURN_VALUE_DISCARDED GDscript warning text to mention how the return value of a function is discarded; update GDScript parser warning test to include new warning text.
This makes sure that assigning values to enum-typed variables are
consistent. Same enum is always valid, different enum is always
invalid (without casting) and assigning `int` creates a warning
if there is no casting.
There are new test cases to ensure this behavior doesn't break in
the future.
