lntemp+itest: refactor node restart process

This commit refactors how the node restart is done. It removes the usage
of `HasSeed` and replaces it with `SkipUnlock` for clarity.

lntest/itest/lnd_channel_backup_test.go

@@ -349,8 +349,9 @@ func testChannelBackupRestoreBasic(ht *lntemp.HarnessTest) {
 						"", revocationWindow, nil,
 						copyPorts(oldNode),
 					)
-
-					st.RestartNode(newNode, backupSnapshot)
+					st.RestartNodeWithChanBackups(
+						newNode, backupSnapshot,
+					)
 
 					return newNode
 				}