Add sample golang app

Signed-off-by: Taylor Silva <tsilva@pivotal.io>

apps/golang/go.mod

@@ -0,0 +1,3 @@
+module github.com/concourse/examples/apps/golang
+
+go 1.16

apps/golang/main.go

@@ -0,0 +1,11 @@
+package main
+
+import "fmt"
+
+func AddSix(i int) int {
+	return i + 6
+}
+
+func main() {
+	fmt.Println("Hello world")
+}

apps/golang/main_test.go

@@ -0,0 +1,11 @@
+package main
+
+import (
+	"testing"
+)
+
+func TestAddSix(t *testing.T) {
+	if AddSix(2) != 8 {
+		t.Error("Expected 2 + 6 to equal 8")
+	}
+}