lntest/timeouts: fix darwin+kvdb_etcd build

Currently trying to run etcd tests on darwin will cause the timeouts to
improperly select timeouts_darwin.go which are stricter than
timeouts_etcd.go. We fix this by always defaulting to timeouts_etcd.go
no matter the platform, and only falling back to timeouts_darwin.go if
the kvdb_etcd tag is not present.

lntest/timeouts_darwin.go

@@ -1,4 +1,4 @@
-// +build darwin
+// +build darwin,!kvdb_etcd
 
 package lntest
 

lntest/timeouts_etcd.go

@@ -1,4 +1,4 @@
-// +build !darwin,kvdb_etcd
+// +build kvdb_etcd
 
 package lntest