crypto/x509: save the temp dir in TestReadUniqueDirectoryEntries

In CL 231958, TempDir was changed to create a new temp directory on
each allocation, on the theory that it is easy to save in a variable
for callers that want the same directory repeatedly. Apply that
transformation here.

Updates #38850

Change-Id: Ibb014095426c33038e0a2c95303579cf95d5c3ba
Reviewed-on: https://go-review.googlesource.com/c/go/+/234582
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
diff --git a/src/crypto/x509/root_unix_test.go b/src/crypto/x509/root_unix_test.go
index 39556ae..5a80154 100644
--- a/src/crypto/x509/root_unix_test.go
+++ b/src/crypto/x509/root_unix_test.go
@@ -204,7 +204,8 @@
 }
 
 func TestReadUniqueDirectoryEntries(t *testing.T) {
-	temp := func(base string) string { return filepath.Join(t.TempDir(), base) }
+	tmp := t.TempDir()
+	temp := func(base string) string { return filepath.Join(tmp, base) }
 	if f, err := os.Create(temp("file")); err != nil {
 		t.Fatal(err)
 	} else {
@@ -216,7 +217,7 @@
 	if err := os.Symlink("../target-out", temp("link-out")); err != nil {
 		t.Fatal(err)
 	}
-	got, err := readUniqueDirectoryEntries(t.TempDir())
+	got, err := readUniqueDirectoryEntries(tmp)
 	if err != nil {
 		t.Fatal(err)
 	}