Fixed the xunit test runner to comply with the latest styles and normal opts

 * support all output types instead of just html
 * accept an output path filename, as expected, and append a count if necessary
 * renamed skip_test_failures to continue_on_error, as it might be generally useful
 * test against mocked command output, not a real execution/logger

xunit has to be a pain: one assembly per run
test the captured command output
allow all output types