Add more tests for multi-precision integers.

tests/basics/int-big-lshift.py

+# tests transition from small to large int representation by left-shift operation
+for i in range(1, 17):
+    for shift in range(70):
+        print(i, '<<', shift, '=', i << shift)

tests/basics/int-big-mul.py

+# tests transition from small to large int representation by multiplication
+for rhs in range(2, 11):
+    lhs = 1
+    for k in range(100):
+        res = lhs * rhs
+        print(lhs, '*', rhs, '=', res)
+        lhs = res
+