sc-consensus-beefy: fix flaky test beefy_reports_equivocations (#14382)

Test was using too small a timeout of 250ms to verify equivocation is reported.

If the test machine is loaded with many async tasks running, the voters work
and subsequent gossiping does not fit within the 250ms window.

Do multiple tries to verify equivocation reporting with exponential timeouts,
most times it will be within 250ms, but the test will retry several times up
to 5seconds total before giving up.

Signed-off-by: Adrian Catangiu <adrian@parity.io>