Support testing of configurations in JSON syntax.

This implements the proposal described at
hashicorp#722 (comment)

helper/resource/testing.go

@@ -414,6 +414,9 @@ type TestStep struct {
 	// Config a string of the configuration to give to Terraform. If this
 	// is set, then the TestCase will execute this step with the same logic
 	// as a `terraform apply`.
+	//
+	// JSON Configuration Syntax can be used and is assumed whenever Config
+	// contains valid JSON.
 	Config string
 
 	// Check is called after the Config is applied. Use this step to

internal/plugintest/working_dir.go

@@ -3,6 +3,7 @@ package plugintest
 import (
 	"bytes"
 	"context"
+	"encoding/json"
 	"errors"
 	"fmt"
 	"io/ioutil"
@@ -14,8 +15,9 @@ import (
 )
 
 const (
-	ConfigFileName = "terraform_plugin_test.tf"
-	PlanFileName   = "tfplan"
+	ConfigFileName     = "terraform_plugin_test.tf"
+	ConfigFileNameJSON = ConfigFileName + ".json"
+	PlanFileName       = "tfplan"
 )
 
 // WorkingDir represents a distinct working directory that can be used for
@@ -28,6 +30,10 @@ type WorkingDir struct {
 	// baseDir is the root of the working directory tree
 	baseDir string
 
+	// configFilename is the full filename where the latest configuration
+	// was stored; empty until SetConfig is called.
+	configFilename string
+
 	// baseArgs is arguments that should be appended to all commands
 	baseArgs []string
 
@@ -83,8 +89,20 @@ func (wd *WorkingDir) GetHelper() *Helper {
 // Destroy to establish the configuration. Any previously-set configuration is
 // discarded and any saved plan is cleared.
 func (wd *WorkingDir) SetConfig(cfg string) error {
-	configFilename := filepath.Join(wd.baseDir, ConfigFileName)
-	err := ioutil.WriteFile(configFilename, []byte(cfg), 0700)
+	if wd.configFilename != "" {
+		err := os.Remove(wd.configFilename)
+		if err != nil && !os.IsNotExist(err) {
+			return err
+		}
+	}
+
+	bCfg := []byte(cfg)
+	if json.Valid(bCfg) {
+		wd.configFilename = filepath.Join(wd.baseDir, ConfigFileNameJSON)
+	} else {
+		wd.configFilename = filepath.Join(wd.baseDir, ConfigFileName)
+	}
+	err := ioutil.WriteFile(wd.configFilename, bCfg, 0700)
 	if err != nil {
 		return err
 	}
@@ -135,17 +153,13 @@ func (wd *WorkingDir) ClearPlan() error {
 // Init runs "terraform init" for the given working directory, forcing Terraform
 // to use the current version of the plugin under test.
 func (wd *WorkingDir) Init() error {
-	if _, err := os.Stat(wd.configFilename()); err != nil {
+	if wd.configFilename == "" {
 		return fmt.Errorf("must call SetConfig before Init")
 	}
 
 	return wd.tf.Init(context.Background(), tfexec.Reattach(wd.reattachInfo))
 }
 
-func (wd *WorkingDir) configFilename() string {
-	return filepath.Join(wd.baseDir, ConfigFileName)
-}
-
 func (wd *WorkingDir) planFilename() string {
 	return filepath.Join(wd.baseDir, PlanFileName)
 }