server/storage/mvcc: deflake TestHashKVWhenCompacting

The HashByRev-goroutines exit since receive `donec` notification. The
Check-computed-hashes goroutine could not have chance to get the hash
result and be stuck forever. We should add case for donec when we wait
for hash result.

Signed-off-by: Wei Fu <fuweid89@gmail.com>

server/storage/mvcc/kvstore_test.go

@@ -591,7 +591,12 @@ func TestHashKVWhenCompacting(t *testing.T) {
 		defer wg.Done()
 		revHash := make(map[int64]uint32)
 		for {
-			r := <-hashCompactc
+			select {
+			case r := <-hashCompactc:
+			case <-donec:
+				return
+			}
+
 			if revHash[r.compactRev] == 0 {
 				revHash[r.compactRev] = r.hash
 			}