test: introduce test script

Why do this? `go test ./...` has a ton of annoying output:

```
?   	github.com/coreos/etcd	[no test files]
?   	github.com/coreos/etcd/crc	[no test files]
?   	github.com/coreos/etcd/elog	[no test files]
?   	github.com/coreos/etcd/error	[no test files]
ok  	github.com/coreos/etcd/etcdserver	0.267s
ok  	github.com/coreos/etcd/etcdserver/etcdhttp	0.022s
?   	github.com/coreos/etcd/etcdserver/etcdserverpb	[no test files]
ok  	github.com/coreos/etcd/raft	0.157s
?   	github.com/coreos/etcd/raft/raftpb	[no test files]
ok  	github.com/coreos/etcd/snap	0.018s
?   	github.com/coreos/etcd/snap/snappb	[no test files]
third_party/code.google.com/p/gogoprotobuf/proto/testdata/test.pb.go🔢
undefined: __emptyarchive__.Extension
ok  	github.com/coreos/etcd/store	4.247s
ok
github.com/coreos/etcd/third_party/code.google.com/p/go.net/context
2.724s
FAIL
github.com/coreos/etcd/third_party/code.google.com/p/gogoprotobuf/proto
[build failed]
ok
github.com/coreos/etcd/third_party/github.com/stretchr/testify/assert
0.013s
ok  	github.com/coreos/etcd/wait	0.010s
ok  	github.com/coreos/etcd/wal	0.024s
?   	github.com/coreos/etcd/wal/walpb	[no test files]
```

And we have no had to manually configure drone.io which I want to avoid:
https://drone.io/github.com/coreos/etcd/admin

test

@@ -0,0 +1,6 @@
+#!/bin/sh
+go test ./wal \
+	./snap \
+	./etcdserver/... \
+	./raft \
+	./store