fix 'invalid cross-device link' error

Previously cleaning golden files would involve moving them into a tmp dir, which could fail
if the tmp dir and testdata dir are not on the same device per #20

```
❯ go test -update                                                                                                                                                         2021-03-16 15:19:44
--- FAIL: Test_WorstScores_nilResult (0.00s)
    autogold.go:82: rename testdata/nil.golden /tmp/go-golden-074934610/nil.golden: invalid cross-device link
FAIL
exit status 1
FAIL    <pkg name redacted>  0.006s
```

This changes the tmp dir we use to be the same path as the testdata dir, plus a `.autogold.tmp`
extension - so it is always on the same device.

Fixes #20

Signed-off-by: Stephen Gutekanst <[email protected]>

README.md

@@ -153,6 +153,7 @@ Additionally, CLI flag behavior has been improved substantially based on experie
 * Previously autogold would fail tests when running `-update`, meaning you may need to run `go test -update` many times to get to your desired end-state if updating a lot of test cases. Now we match the behavior of OCaml expect tests in not failing tests by default (you can now specify `-fail-on-update`)
 * `-fail-on-update` now uses `t.FailNow()` instead of `t.Fatal()` to allow as many tests as possible to succeed when performing an update operation.
 * `autogold.Want` has been deprecated in favor of `autogold.Expect`
+* Fixed `invalid cross-device link` errors on some systems.
 
 Finally, please note the renaming of functions before and after:
 

autogold.go

@@ -72,7 +72,7 @@ func ExpectFile(t *testing.T, got interface{}, opts ...Option) {
 
 	if shouldCleanup() {
 		cleanMu.Lock()
-		if err := mkTempDir(); err != nil {
+		if err := mkTempDir(dir); err != nil {
 			t.Fatal(err)
 		}
 		grabLock()
@@ -231,27 +231,25 @@ func shouldCleanup() bool {
 	return true
 }
 
-func mkTempDir() error {
+func mkTempDir(testDataDir string) error {
 	if cleanDir != "" {
 		return nil
 	}
 
-	// Try to remove past go-golden temp dirs.
-	matches, err := filepath.Glob(filepath.Join(os.TempDir(), "go-golden-*"))
-	if err != nil {
+	// The tmp dir will be `testdata/foo.autogold.tmp` or `testdata.autogold.tmp` (depending on testdata
+	// folder name.)
+	tmpDir := testDataDir + ".autogold.tmp"
+
+	// The dir may have been left behind if a past test run exited early, so remove it.
+	if err := os.RemoveAll(tmpDir); err != nil && !os.IsNotExist(err) {
 		return err
 	}
-	for _, match := range matches {
-		if err := os.RemoveAll(match); err != nil {
-			return err
-		}
-	}
 
-	// Create a temp dir for this run.
-	cleanDir, err = ioutil.TempDir("", "go-golden-*")
-	if err != nil {
+	// Create the tmp dir.
+	if err := os.Mkdir(tmpDir, os.ModePerm); err != nil {
 		return err
 	}
+	cleanDir = tmpDir
 	return nil
 }