docs/devel: rationalise unstable gitlab tests under FLAKY_TESTS

docs/devel: rationalise unstable gitlab tests under FLAKY_TESTS

It doesn't make sense to have two classes of flaky tests. While it may
take the constrained environment of CI to trigger failures easily it
doesn't mean they don't occasionally happen on developer machines. As
CI is the gating factor to passing there is no point developers
running the tests locally anyway unless they are trying to fix things.

While we are at it update the language in the docs to discourage the
QEMU_TEST_FLAKY_TESTS becoming a permanent solution.

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Acked-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20231201093633.2551497-3-alex.bennee@linaro.org>