If something in AfterEach hangs till test timeout, the
coredump produced might covers up the actual assertion
failure in test body. This CR fixes this in etcdraft UT.

This CR also adds LongEventualTimeout to some places in
etcdraft chain_test to prevent flakes due to slow wal sync.

Change-Id: I585e59e5eb587f9e9eb082c5eb8f681141b16e55
Signed-off-by: Jay Guo <guojiannan1101@gmail.com>