Add e2e test for provisioner plugin

Ensure we can still execute a 3rd party provisioner binary, using the
legacy plugin discovery location of the configuration root directory.

command/e2etest/provisioner_plugin_test.go

@@ -0,0 +1,65 @@
+package e2etest
+
+import (
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+
+	"github.com/hashicorp/terraform/e2e"
+)
+
+// TestProviderDevOverrides is a test that terraform can execute a 3rd party
+// provisioner plugin.
+func TestProvisionerPlugin(t *testing.T) {
+	t.Parallel()
+
+	// This test reaches out to releases.hashicorp.com to download the
+	// template and null providers, so it can only run if network access is
+	// allowed.
+	skipIfCannotAccessNetwork(t)
+
+	tf := e2e.NewBinary(terraformBin, "testdata/provisioner-plugin")
+	defer tf.Close()
+
+	// In order to do a decent end-to-end test for this case we will need a
+	// real enough provisioner plugin to try to run and make sure we are able
+	// to actually run it. Here will build the local-exec provisioner into a
+	// binary called test-provisioner
+	provisionerExePrefix := filepath.Join(tf.WorkDir(), "terraform-provisioner-test_")
+	provisionerExe := e2e.GoBuild("github.com/hashicorp/terraform/internal/provisioner-local-exec/main", provisionerExePrefix)
+
+	// provisioners must use the old binary name format, so rename this binary
+	newExe := filepath.Join(tf.WorkDir(), "terraform-provisioner-test")
+	if _, err := os.Stat(newExe); !os.IsNotExist(err) {
+		t.Fatalf("%q already exists", newExe)
+	}
+	if err := os.Rename(provisionerExe, newExe); err != nil {
+		t.Fatalf("error renaming provisioner binary: %v", err)
+	}
+	provisionerExe = newExe
+
+	t.Logf("temporary provisioner executable is %s", provisionerExe)
+
+	//// INIT
+	_, stderr, err := tf.Run("init")
+	if err != nil {
+		t.Fatalf("unexpected init error: %s\nstderr:\n%s", err, stderr)
+	}
+
+	//// PLAN
+	_, stderr, err = tf.Run("plan", "-out=tfplan")
+	if err != nil {
+		t.Fatalf("unexpected plan error: %s\nstderr:\n%s", err, stderr)
+	}
+
+	//// APPLY
+	stdout, stderr, err := tf.Run("apply", "tfplan")
+	if err != nil {
+		t.Fatalf("unexpected apply error: %s\nstderr:\n%s", err, stderr)
+	}
+
+	if !strings.Contains(stdout, "HelloProvisioner") {
+		t.Fatalf("missing provisioner output:\n%s", stdout)
+	}
+}

command/e2etest/testdata/provisioner-plugin/main.tf

@@ -0,0 +1,5 @@
+resource "null_resource" "a" {
+  provisioner "test" {
+    command = "echo HelloProvisioner"
+  }
+}