tests/basics: Add test constructing a set from a non-trivial expression.

tests/basics/set1.py

@@ -6,6 +6,10 @@ print(s)
 s = {3, 4, 3, 1}
 print(sorted(s))
 
+# expression in constructor
+s = {1 + len(s)}
+print(s)
+
 # Sets are not hashable
 try:
     {s: 1}