Preserve log output by test into logs/ directory.

This feature enables logging output of each Test into "logs/$roundup_p-${roundup_test_name}.log" file.

For example I have about 100 tests for a single project and roundup is running via crond on a nightly basis.
Idea is to send generic report about PASS and FAIL tests via email (roundup stdout)
and put log filename nearby.

This feature is only enabled if "logs/" directory exists in the current dir and turned off by default.

Output example:
00-mkproxy-config
  it_has_mkproxy_home:      [PASS] logs/00-mkproxy-config-test.sh-it_has_mkproxy_home.log
  it_uses_profie_config:        [PASS] logs/00-mkproxy-config-test.sh-it_uses_profie_config.log
  it_found_java:                     [PASS] logs/00-mkproxy-config-test.sh-it_found_java.log
  it_found_java_version:                           [PASS] logs/00-mkproxy-config-test.sh-it_found_java_version.log
  it_found_uberjar:                                [PASS] logs/00-mkproxy-config-test.sh-it_found_uberjar.log
  it_found_static_bins:                            [FAIL] logs/00-mkproxy-config-test.sh-it_found_static_bins.log
  it_found_python_depenencies:                     [PASS] logs/00-mkproxy-config-test.sh-it_found_python_depenencies.log