objset: Add frozenset tests, skippable if frozenset not available.

tests/basics/frozenset1.py

@@ -0,0 +1,14 @@
+# basic sets
+
+try:
+    frozenset
+except NameError:
+    print("SKIP")
+    import sys
+    sys.exit()
+
+s = frozenset({1})
+print(s)
+
+s = frozenset({3, 4, 3, 1})
+print(sorted(s))