docs: add definitions of terms for CI/testing

docs: add definitions of terms for CI/testing

To understand the current state of QEMU CI/testing and have a base to
discuss the plans for the future, it is important to define some usual
terms. This patch defines the terms for "Automated tests", "Unit
testing", "Functional testing", "System testing", "Flaky tests",
"Gating", and "Continuous Integration".

Signed-off-by: Willian Rampazzo <willianr@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20210831152939.97570-2-willianr@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>