jtreg re-compiles test libraries even when it is not necessary.

If we have a test with the following script (just an example):

 * @library ../common
 * @build jdk.lib.*

and we run this twice in othervm mode, then the files in jdk.lib will be compiled twice, although by the second invocation the files already exists in the work directory. The same thing happens if two tests in the same run uses the same library. This impacts run times severely.