entc/gen: add enttest package for generated code. (#442)

* entc/gen: add enttest package for generated code.

It's still WIP and another 2 things need to be added:
- Add an option to skip schema migration and pass client options
- Update docs and replace existing testing code to use enttest

* entc/gen: finish enttest package tempalte

Signed-off-by: Alex Snast <alexsn@fb.com>

Co-authored-by: Alex Snast <alexsn@fb.com>

examples/start/ent/enttest/enttest.go

@@ -0,0 +1,82 @@
+// Copyright (c) Facebook, Inc. and its affiliates. All Rights Reserved.
+// This source code is licensed under the Apache 2.0 license found
+// in the LICENSE file in the root directory of this source tree.
+
+// Code generated by entc, DO NOT EDIT.
+
+package enttest
+
+import (
+	"context"
+
+	"github.com/facebookincubator/ent/dialect/sql/schema"
+	"github.com/facebookincubator/ent/examples/start/ent"
+
+	// required by schema hooks.
+	_ "github.com/facebookincubator/ent/examples/start/ent/runtime"
+)
+
+type (
+	// TestingT is the interface that is shared between
+	// testing.T and testing.B and used by enttest.
+	TestingT interface {
+		FailNow()
+		Error(...interface{})
+	}
+
+	// Option configures client creation.
+	Option func(*options)
+
+	options struct {
+		opts        []ent.Option
+		migrateOpts []schema.MigrateOption
+	}
+)
+
+// WithOptions forwards options to client creation.
+func WithOptions(opts ...ent.Option) Option {
+	return func(o *options) {
+		o.opts = append(o.opts, opts...)
+	}
+}
+
+// WithMigrateOptions forwards options to auto migration.
+func WithMigrateOptions(opts ...schema.MigrateOption) Option {
+	return func(o *options) {
+		o.migrateOpts = append(o.migrateOpts, opts...)
+	}
+}
+
+func newOptions(opts []Option) *options {
+	o := &options{}
+	for _, opt := range opts {
+		opt(o)
+	}
+	return o
+}
+
+// Open calls ent.Open and auto-run migration.
+func Open(t TestingT, driverName, dataSourceName string, opts ...Option) *ent.Client {
+	o := newOptions(opts)
+	c, err := ent.Open(driverName, dataSourceName, o.opts...)
+	if err != nil {
+		t.Error(err)
+		t.FailNow()
+	}
+	if err := c.Schema.Create(context.Background(), o.migrateOpts...); err != nil {
+		t.Error(err)
+		t.FailNow()
+	}
+	return c
+}
+
+// NewClient calls ent.NewClient and auto-run migration.
+func NewClient(t TestingT, opts ...Option) *ent.Client {
+	o := newOptions(opts)
+	c := ent.NewClient(o.opts...)
+	if err := c.Schema.Create(context.Background(), o.migrateOpts...); err != nil {
+		t.Error(err)
+		t.FailNow()
+	}
+	return c
+}