move tests to underscore prefix so that go get builds successfully

Prior to making these changes, `go get` would fail to build the pixelslinger executable. (using go 1.4)

```
$ go get github.com/longears/pixelslinger
# github.com/longears/pixelslinger
.go/src/github.com/longears/pixelslinger/pixelslinger.go:234: main redeclared in this block
	previous declaration at .go/src/github.com/longears/pixelslinger/miditest.go:8
.go/src/github.com/longears/pixelslinger/servertest.go:8: main redeclared in this block
	previous declaration at .go/src/github.com/longears/pixelslinger/pixelslinger.go:234
```

Discovered you could prevent them being built using an underscore prefix here: https://github.com/lucasb-eyer/go-colorful/issues/5