Several tests in the suite use large amounts of traffic to e.g. cause
congestion and evaluate RED or shaper performance. These tests will not run
well on a slow machine, be it one with heavy debug kernel, or a VM, or e.g.
a single-board computer. Allow users to specify an environment variable,
KSFT_MACHINE_SLOW=yes, to indicate that the tests are being run on one such
machine.

Performance sensitive tests can then use a new helper, xfail_on_slow(), to
mark parts of the test that are sensitive to low-performance machines.
The helper can be used to just mark the whole suite, like so:

	xfail_on_slow tests_run

... or, on the other side of the granularity spectrum, to override
individual checks:

	xfail_on_slow check_err $? "Expected much, got little."
Signed-off-by: Petr Machata <petrm@nvidia.com>
Link: https://lore.kernel.org/r/99a376a2d2ffdaeee7752b1910cb0c3ea5d80fbe.1711464583.git.petrm@nvidia.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>