tests/avocado: mark boot_linux.py long runtime instead of flaky

tests/avocado: mark boot_linux.py long runtime instead of flaky

The ppc64 and s390x tests were first marked skipIf GITLAB_CI by commit
c0c8687ef0f ("tests/avocado: disable BootLinuxPPC64 test in CI"), and
commit 0f26d94ec9e ("tests/acceptance: skip s390x_ccw_vrtio_tcg on
GitLab") due to being very heavy-weight for gitlab CI.

Commit 9b45cc99318 ("docs/devel: rationalise unstable gitlab tests under
FLAKY_TESTS") changed this to being flaky but it isn't really, it just
had a long runtime.

So take the SPEED=slow variable from qtests and introduce it to avocado,
and make these tests require it.

Reviewed-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>