Fix Go tests to run in module mode

Stop using relative imports and add a go.mod file.

Tested against Go 1.6 through Go pre-1.13, and gccgo.

Examples/test-suite/go/constover_runme.go

@@ -1,9 +1,10 @@
 package main
 
 import (
-	"./constover"
 	"fmt"
 	"os"
+
+	"constover"
 )
 
 func main() {