Use TemporaryFolder to good effect.

Prevents us from leaving behind a temp dir with cyclic symlinks, which breaks JGit’s clean function.
That in turn can cause test builds to break:
https://jenkins.ci.cloudbees.com/job/core/job/jenkins_main_trunk/783/console
Cleaning workspace
FATAL: Could not delete file …/workspace/core/jenkins_main_trunk/core/target/hudson5309354136960541268tmp/c/symlink/c/symlink/c/symlink/…/c/foo.xml
org.eclipse.jgit.api.errors.JGitInternalException: Could not delete file …/workspace/core/jenkins_main_trunk/core/target/hudson5309354136960541268tmp/c/symlink/c/symlink/c/symlink/…/c/foo.xml
	at org.eclipse.jgit.api.CleanCommand.call(CleanCommand.java:137)
	at org.jenkinsci.plugins.gitclient.JGitAPIImpl.clean(JGitAPIImpl.java:305)
	at hudson.plugins.git.GitAPI.clean(GitAPI.java:248)
	at hudson.plugins.git.GitSCM$4.invoke(GitSCM.java:1213)
	at hudson.plugins.git.GitSCM$4.invoke(GitSCM.java:1196)
	at hudson.FilePath$FileCallableWrapper.call(FilePath.java:2236)
	at hudson.remoting.UserRequest.perform(UserRequest.java:118)
	at hudson.remoting.UserRequest.perform(UserRequest.java:48)
	at hudson.remoting.Request$2.run(Request.java:326)
	at hudson.remoting.InterceptingExecutorService$1.call(InterceptingExecutorService.java:72)
	at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:334)
	at java.util.concurrent.FutureTask.run(FutureTask.java:166)
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
	at java.lang.Thread.run(Thread.java:722)
Caused by: java.io.IOException: Could not delete file …/workspace/core/jenkins_main_trunk/core/target/hudson5309354136960541268tmp/c/symlink/c/symlink/c/symlink/…/c/foo.xml
	at org.eclipse.jgit.util.FileUtils.delete(FileUtils.java:142)
	at org.eclipse.jgit.util.FileUtils.delete(FileUtils.java:126)
	at org.eclipse.jgit.util.FileUtils.delete(FileUtils.java:126)
	…
	at org.eclipse.jgit.util.FileUtils.delete(FileUtils.java:126)
	at org.eclipse.jgit.util.FileUtils.delete(FileUtils.java:126)
	at org.eclipse.jgit.api.CleanCommand.call(CleanCommand.java:132)
	... 14 more

core/src/test/java/hudson/util/SecretRewriterTest.groovy

@@ -5,11 +5,10 @@ import hudson.FilePath
 import jenkins.security.ConfidentialStoreRule
 import org.junit.Rule
 import org.junit.Test
+import org.junit.rules.TemporaryFolder
 
 import javax.crypto.Cipher
 
-import static hudson.Util.createTempDir
-
 /**
  *
  *
@@ -22,6 +21,8 @@ class SecretRewriterTest {
     @Rule
     public ConfidentialStoreRule confidentialStoreRule = new ConfidentialStoreRule();
 
+    @Rule public TemporaryFolder tmp = new TemporaryFolder()
+
     @Test
     void singleFileRewrite() {
         def o = encryptOld('foobar') // old
@@ -48,14 +49,10 @@ class SecretRewriterTest {
 
     void roundtrip(String before, String after) {
         def sr = new SecretRewriter(null);
-        def f = File.createTempFile("test","xml");
-        try {
-            f.text = before
-            sr.rewrite(f,null)
-            assert after.trim()==f.text.trim()
-        } finally {
-            f.delete()
-        }
+        def f = File.createTempFile("test", "xml", tmp.root)
+        f.text = before
+        sr.rewrite(f,null)
+        assert after.trim()==f.text.trim()
     }
 
     String encryptOld(str) {
@@ -73,7 +70,7 @@ class SecretRewriterTest {
      */
     @Test
     void recursionDetection() {
-        def backup = createTempDir()
+        def backup = tmp.newFolder("backup")
         def sw = new SecretRewriter(backup);
         def st = StreamTaskListener.fromStdout()
 
@@ -83,7 +80,7 @@ class SecretRewriterTest {
         def answer = "<msg>$n</msg>"
 
         // set up some directories with stuff
-        def t = createTempDir()
+        def t = tmp.newFolder("t")
         def dirs = ["a", "b", "c", "c/d", "c/d/e"]
         dirs.each { p ->
             def d = new File(t, p)
@@ -92,7 +89,7 @@ class SecretRewriterTest {
         }
 
         // stuff outside
-        def t2 = createTempDir()
+        def t2 = tmp.newFolder("t2")
         new File(t2,"foo.xml").text = payload
 
         // some recursions as well as valid symlinks