wal: add test SegmentMetaTest.EncodeAndDecode.
wal: add test SegmentMetaTest.EncodeAndDecode.

neverchanje/yaraft-wal,neverchanje/consensus-yaraft,neverchanje/consensus-yaraft,neverchanje/yaraft-wal,neverchanje/consensus-yaraft,neverchanje/consensus-yaraft,neverchanje/yaraft-wal