oeqa/selftest: Hardcode test assumptions about heartbeat event timings

Setting a value of 10 for heartbeat events causes the test to fail. Hardcode
a value to ensure it works correctly even if the default is changed.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 08b2c9a2)
Signed-off-by: Steve Sakoman <steve@sakoman.com>