Unit tests

```
$ go test -cover -race ./...                                                              (328ms) 
ok  	github.com/SentimensRG/ctx	1.035s	coverage: 73.0% of statements
ok  	github.com/SentimensRG/ctx/mergectx	1.711s	coverage: 54.0% of statements
ok  	github.com/SentimensRG/ctx/refctx	1.032s	coverage: 75.0% of statements
?   	github.com/SentimensRG/ctx/sigctx	[no test files]
```