Tests: reworked try_run() to redirect stderr. Previously, if TEST_NGINX_VERBOSE was set, the code tried to use compiled-in error log. While this works in typical development cases when --error-log-path is relative to prefix, it is unlikely to work correctly if --error-log-path is absolute. The recently introduced '-e' command line option might be used to fix this. On the other hand, a more compatible solution would be to redirect stderr to a file instead.