test: return cleanup function for temp plugin dirs

When creating a temporary plugin directory, we had to build a cleanup
function, which could be as simple as `os.Mkdirall` without any kind of
warning that a directory failed to be cleaned-up, or we could do some
more work in order to report the possible issues around this.

That code would quickly be redundant, as there's not a ton of
variability in the code that can be written for this step, so we
abstract it through a pre-defined cancellation function which can be
safely defer invoked.

test/loading_test.go

@@ -1,20 +1,14 @@
 package test
 
 import (
-	"os"
 	"testing"
 )
 
 func (ts *PackerTestSuite) TestLoadingOrder() {
 	t := ts.T()
 
-	pluginDir := ts.MakePluginDir(t, "1.0.9", "1.0.10")
-	defer func() {
-		err := os.RemoveAll(pluginDir)
-		if err != nil {
-			t.Logf("failed to remove temporary plugin directory %q: %s. This may need manual intervention.", pluginDir, err)
-		}
-	}()
+	pluginDir, cleanup := ts.MakePluginDir(t, "1.0.9", "1.0.10")
+	defer cleanup()
 
 	for _, command := range []string{"build", "validate"} {
 		tests := []struct {

test/plugin_test.go

@@ -2,6 +2,7 @@ package test
 
 import (
 	"fmt"
+	"io"
 	"os"
 	"os/exec"
 	"path/filepath"
@@ -160,7 +161,7 @@ func currentDir() (string, error) {
 // packer will be able to use that directory for running its functions.
 //
 // Deletion of the directory is the caller's responsibility.
-func (ts *PackerTestSuite) MakePluginDir(t *testing.T, pluginVersions ...string) (pluginTempDir string) {
+func (ts *PackerTestSuite) MakePluginDir(t *testing.T, pluginVersions ...string) (pluginTempDir string, cleanup func()) {
 	var err error
 
 	defer func() {
@@ -188,5 +189,10 @@ func (ts *PackerTestSuite) MakePluginDir(t *testing.T, pluginVersions ...string)
 		}
 	}
 
-	return pluginTempDir
+	return pluginTempDir, func() {
+		err := os.RemoveAll(pluginTempDir)
+		if err != nil {
+			t.Logf("failed to remove temporary plugin directory %q: %s. This may need manual intervention.", pluginTempDir, err)
+		}
+	}
 }