test: Add test for SIP-62 for

**Problem/Solution**
This add a test for SIP-62, which allows for comprehension that starts with an alias.

Author: eed3si9n

test/corpus/expressions.txt

@@ -1473,6 +1473,7 @@ def f(): Unit =
   for n <- nums yield n + 1
 
   for
+    a = 1
     x <- xs
     y <- ys
   yield
@@ -1502,6 +1503,9 @@ def f(): Unit =
           (integer_literal)))
       (for_expression
         (enumerators
+          (enumerator
+            (identifier)
+            (integer_literal))
           (enumerator
             (identifier)
             (identifier))