internal/pkg: tar optional file

This allows tar to take a single-file directory Artifact as input.

Signed-off-by: Ophestra <cat@gensokyo.uk>

internal/pkg/exec_test.go

@@ -300,26 +300,6 @@ func newTesttool() (
 			).String(), testtoolBin, 0500)
 		},
 	}}
-	testtoolDestroy = func(t *testing.T, base *check.Absolute, c *pkg.Cache) {
-		if pathname, checksum, err := c.Cure(testtool); err != nil {
-			t.Fatalf("Cure: error = %v", err)
-		} else if err = os.Remove(pathname.String()); err != nil {
-			t.Fatal(err)
-		} else {
-			p := base.Append(
-				"checksum",
-				pkg.Encode(checksum),
-			)
-			if err = os.Chmod(p.Append("bin").String(), 0700); err != nil {
-				t.Fatal(err)
-			}
-			if err = os.Chmod(p.String(), 0700); err != nil {
-				t.Fatal(err)
-			}
-			if err = os.RemoveAll(p.String()); err != nil {
-				t.Fatal(err)
-			}
-		}
-	}
+	testtoolDestroy = newDestroyArtifactFunc(testtool)
 	return
 }