tests/acceptance: Skip slow quanta-gsj U-boot+Linux test

tests/acceptance: Skip slow quanta-gsj U-boot+Linux test

The 'arm_quanta_gsj_initrd' test is timeouting on GitLab CI:
https://gitlab.com/philmd/qemu/-/jobs/745483978#L846
and also sometimes on my workstation, so proceed as with
the other slow tests: do not run it by default.
The test can still be run setting the AVOCADO_TIMEOUT_EXPECTED
environment variable.

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Willian Rampazzo <willianr@redhat.com>
Reviewed-by: Havard Skinnemoen <hskinnemoen@google.com>
Tested-by: Havard Skinnemoen <hskinnemoen@google.com>
Message-Id: <20200918143355.153522-1-f4bug@amsat.org>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>