[issue5663] Better failure messages for unittest assertions

Patch for unittest on trunk.

It provides better default failure messages for assertTrue and
assertFalse (current is "AssertionError: None").

It also provides a new class attribute for TestCase: longMessage

This defaults to False. If set to True, passing in an explicit custom
message does *not* override the helpful default failure message in
asserts which tell you which objects were involved in the failure.

Even if set to True or False in a TestCase then longMessage can still be
overridden in individual tests by setting an instance attribute.



Needs docs.

Could longMessage default to True in 3.1?