Fix TemporaryFolderUsageTest on Windows (junit-team#1428)

Fix newFolderWithPathStartingWithFileSeparatorThrowsIOException Test run on Windows OS

Author: powazny

src/test/java/org/junit/rules/TemporaryFolderUsageTest.java

@@ -106,7 +106,7 @@ public void newFolderWithPathStartingWithFileSeparatorThrowsIOException()
         }
         tempFolder.create();
         thrown.expect(IOException.class);
-        thrown.expectMessage("folder path '/temp1' is not a relative path");
+        thrown.expectMessage("folder path '" + fileAtRoot + "' is not a relative path");
         tempFolder.newFolder(fileAtRoot);
     }