linux-kselftest-5.4-rc1

This Kselftest update for Linux 5.4-rc1 consists of several fixes to
existing tests and adds KUnit, a lightweight unit testing and mocking
framework for the Linux kernel from Brendan Higgins.

KUnit is not an end-to-end testing framework. It is currently supported
on UML and sub-systems can write unit tests and run them in UML env.
KUnit documentation is included in this update.

In the future KUnit will be linked to Kselftest framework to provide
a way to trigger KUnit tests from user-space.