Make ShardTest.testCommitWhenTransactionHasModifications() wait a bit
Committed transactions involve also a purge payload, which is persisted
asynchronously, hence it may or may not be visible in the journal just
after the transaction is reported as committed. Wait for two heartbeat
intervals before looking at the stats.
Change-Id: Ibe699edced12d006bf5ea8cd99aa821ab56d115d
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>
(cherry picked from commit
3e86a2e5c83a86daba1afa8eeabc14c060c7f454)