Odoo Test environments requires to modify many parts of the unittest
TestCase, Suite and Result.

The main initial reason is to **avoid to postpone result at the end of
the test suite**, because even if it is convenient to have all errors
visible after the tests in some case, odoo logs adds information during
the execution that can be useful to debug when a test fail, to have
context for an error. (see **OdooTestResult**)

We are also fixing the stack trace comming from a unittest and since
there is no proper way to hook inside the TestPartExecutor, a dirty hack
injects anoter result on the outcome to manage the error and complete
the stack trace. This was also a way to avoid to postpone subtest logs
at the end of the test case (see _ErrorCatcher)

`_feedErrorsToResult` was used to test the test suite behavior since
there are many customization and this is quite fragile, especially if
unittest changes behavior in other python version.

**Python 3.11** introduced python/cpython#664448d8 That, in a way, goes
in the same direction of the changed introduced with _ErrorCatcher:
immediately feed errors to resut instead of postponing it. But this also
removes `_feedErrorsToResult` that was used to test this behaviors, as
well as other ones.

Since odoo should remain multi-version, this amount of changes on the
initial behavior become to complicate to keep cross-version and the
(already in our mind for a while) solution to **vendor unittest** will
help to simplify most of our test code base.

This commit modified the vendored unittest files to simplify them as
much as possible to suite our needs.

Since the runner is still the unittest one, we need to inherit from
unittest.Testcase in order to have the right type.

This also means that we still have access to all TestCase methods
without overriding them all. This is convenient for assertion methods as
an example but the initial idea is to vendor our own version of TestCase
to avoid having trouble to adapte our miscommunications to future python
versions. A trade-off must be done to chose what should remain in our
code base. The idea is to keep logic closely linked to our changes in
our code base, mainly around the run method, but also addClassCleanup
wich need to be vendored for python 3.7, but assertions methods are
independent. Any logic can be moved fom unittest to our
vendored version in the future if needed.

Part-of: odoo/odoo#112294