RFR(T) 8249586: Test runtime/cds/appcds/DirClasspathTest.java will fail if run twice

[Calvin Cheung]

bug: https://bugs.openjdk.java.net/browse/JDK-8249586

webrev: http://cr.openjdk.java.net/~ccheung/jdk16/8249586/webrev.00/

A trivial fix by adding the StandardCopyOption.REPLACE_EXISTING option 
when calling Files.copy().

Tested by running the jtreg test multiple times in the same directory.

thanks,

Calvin