Add tests for issue #599

Add four separate tests with prefix of sys_path for four types of
sys.path entries: str, unicode, subclass of str, and subclass of
unicode.

test/tests/sys_path_str.py

+import sys
+sys.path.append("test_package")
+import import_target

test/tests/sys_path_str_subclass.py

+import sys
+class TestClass(str):
+    pass
+t = TestClass("test_package")
+sys.path.append(t)
+import import_target

test/tests/sys_path_unicode.py

+import sys
+sys.path.append(u"test_package")
+import import_target

test/tests/sys_path_unicode_subclass.py

+import sys
+class TestClass(unicode):
+    pass
+t = TestClass("test_package")
+sys.path.append(t)
+import import_target