selftests: openvswitch: retry instead of sleep

selftests: openvswitch: retry instead of sleep

There are a couple of places where the test script "sleep"s to wait for
some external condition to be met.

This is error prone, specially in slow systems (identified in CI by
"KSFT_MACHINE_SLOW=yes").

To fix this, add a "ovs_wait" function that tries to execute a command
a few times until it succeeds. The timeout used is set to 5s for
"normal" systems and doubled if a slow CI machine is detected.

This should make the following work:

$ vng --build  \
    --config tools/testing/selftests/net/config \
    --config kernel/configs/debug.config

$ vng --run . --user root -- "make -C tools/testing/selftests/ \
    KSFT_MACHINE_SLOW=yes TARGETS=net/openvswitch run_tests"

Signed-off-by: Adrian Moreno <amorenoz@redhat.com>
Reviewed-by: Ilya Maximets <i.maximets@ovn.org>
Link: https://patch.msgid.link/20240710090500.1655212-1-amorenoz@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>