Add Go build support for GN.

mojo/go/tests/system_test.go

+package tests
+
+import "mojo/go/system/embedder"
+import "mojo/go/system/impl"
+import "mojo/public/go/mojo/system"
+import "testing"
+
+func Init() {
+        embedder.InitializeMojoEmbedder();
+}
+
+func TestGetTimeTicksNow(t *testing.T) {
+        Init();
+        var c system.Core = impl.GetCore();
+        x := c.GetTimeTicksNow();
+        if (x < 10) {
+                t.Error("hi");
+        }
+}