tests/avocado: Disable the test_sbsaref_edk2_firmware by default

tests/avocado: Disable the test_sbsaref_edk2_firmware by default

The test fails occasionally, see e.g.:

 https://gitlab.com/thuth/qemu/-/jobs/4196177756#L489
 https://gitlab.com/thuth/qemu/-/jobs/4623296271#L290

It also fails on my laptop in ca. 50% of all runs. Thus disable it by
default by using the QEMU_TEST_FLAKY_TESTS environment variable to fence
it (which we also already use in flaky qtests). While we're at it, also
document this variable in docs/devel/testing.rst.

Message-Id: <20230710170155.7192-1-thuth@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>