tests/import: Add a test for the builtin __import__ function.

tests/import/builtin_import.py

+# test calling builtin import function
+
+# basic test
+__import__('builtins')
+
+# first arg should be a string
+try:
+    __import__(1)
+except TypeError:
+    print('TypeError')
+
+# level argument should be non-negative
+try:
+    __import__('xyz', None, None, None, -1)
+except ValueError:
+    print('ValueError')