Allow prefix removal in junit reports

Some invocations of start_test/sub_test result in sub_test using absolute
paths in the test results. This results in really long class names in jUnit
report, not to mention it makes the results less comparable from one run
to the next if the test location changes on a file system.

Update the converter to accept a prefix that will be removes the class name
of all test cases. Pipe the feature through start_test and paratest.server.
The default is to not do any prefixing (start_test doesn't necessarily know
what should be removed in the general case).