doc: add documentation and example for enttest (#453)

doc/md/testing.md

@@ -0,0 +1,39 @@
+---
+id: testing
+title: Testing
+---
+
+If you're using `ent.Client` in your unit-tests, you can use the generated `enttest`
+package for creating a client and auto-running the schema migration as follows:
+
+```go
+package main
+
+import (
+	"testing"
+
+	"<project>/ent/enttest"
+
+	_ "github.com/mattn/go-sqlite3"
+)
+
+func TestXXX(t *testing.T) {
+	client := enttest.Open(t, "sqlite3", "file:ent?mode=memory&cache=shared&_fk=1")
+	defer client.Close()
+    // ...
+}
+```
+
+In order to pass functional options to `Open`, use `enttest.Option`:
+
+```go
+func TestXXX(t *testing.T) {
+    opts := []enttest.Option{
+    	enttest.WithOptions(ent.Log(t.Log)),
+        enttest.WithMigrateOptions(migrate.WithGlobalUniqueID(true)),
+    }
+	client := enttest.Open(t, "sqlite3", "file:ent?mode=memory&cache=shared&_fk=1", opts...)
+	defer client.Close()
+    // ...
+}
+```