tests/functional: Add a test for the Stellaris arm machines

tests/functional: Add a test for the Stellaris arm machines

The 2023 edition of the QEMU advent calendar featured an image
that we can use to test whether the lm3s6965evb machine is basically
still working.

And for the lm3s811evb there is a small test kernel on github
which can be used to check its UART.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-id: 20250519170242.520805-1-thuth@redhat.com
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>