docs/system/arm: Use "functional tests" instead of "integration tests"

docs/system/arm: Use "functional tests" instead of "integration tests"

We don't use the term "integration tests" for these kind of tests
anymore, it's "functional tests" nowadays.

Suggested-by: Niek Linnenbank <nieklinnenbank@gmail.com>
Message-ID: <20250318061420.20378-1-thuth@redhat.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Niek Linnenbank <nieklinnenbank@gmail.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>