Separation of re-usable code in test.drush.inc into extra function.

Moved compilation of test results from drush_test_xml_results()
into a separate function drush_test_get_results().
Thus the data structure $test_suite, which contains all information
about the test results, can be used for otehr reports as well.
Merge pull request #92 from DirkR/separate_testresults_and_xmlbuild

Move the compilation of test results from drush_test_xml_results() into a separate function drush_test_get_results().