gistfile1.txt

$ cat main.go
package main

var x = "abc"

func main() {
        println(x)
}

$ cat alt.go
// +build testing

package main

func init() {
        x = "def"
}

$ go build
$ ./xxx
abc

$ rm ./xxx
$ go build -tags=testing
$ ./xxx
def

$ rm ./xxx
$ go build -tags=testing ; go build ; ./xxx
abc