Make arc unit NoseTestEngine work in a sub-dir

Summary:
D14362 didn't actually work in a subdirectory.

This globs the tests/ directory to find test_*.py, now you get
more than just a coverage report when running `arc unit --everything`
whether it's run from the project root or a subdirectory.

Test Plan:
ran `arc unit --everything` from project root and also ran it from
a sub-directory.

```
$ arc unit --everything
   PASS    1ms★  test_log.FilterTest.test_filter_can_invert_behavior
   PASS    1ms★  test_log.FilterTest.test_filter_filters_matching
   PASS   <1ms★  test_log.FilterTest.test_filter_filters_matching_lambda
   PASS    1ms★  test_log.FilterTest.test_filter_filters_matching_regex
   PASS    3ms★  test_log.FilterTest.test_filter_loads
   PASS    2ms★  test_log.FilterTest.test_filter_loads_filters_correctly
   PASS   <1ms★  test_log.FilterTest.test_filter_loads_supports_numeric_comparisons
   PASS    1ms★  test_log.FilterTest.test_filter_parse
   PASS    1ms★  test_log.JSONFormatterTest.test_format_includes_all_serializable_logrecord_fields
   PASS    1ms★  test_log.JSONFormatterTest.test_format_includes_exceptions_as_text
   PASS   <1ms★  test_log.JSONFormatterTest.test_format_includes_extra_fields
   PASS    1ms★  test_log.JSONFormatterTest.test_make_record
   PASS    1ms★  test_nrpe.NRPETest.test_load
   PASS   <1ms★  test_nrpe.NRPETest.test_registered_check
   PASS   <1ms★  test_nrpe.NRPETest.test_unregistered_check
   PASS   <1ms★  test_ssh.JSONOutputHandlerTest.test_accept_ignores_bad_json
   PASS   <1ms★  test_ssh.JSONOutputHandlerTest.test_accept_parses_and_logs_json_messages
   PASS   <1ms★  test_ssh.JSONOutputHandlerTest.test_lines_buffers_partial_lines
   PASS    4ms★  test_checks.ChecksConfigTest.test_custom_type
   PASS    2ms★  test_checks.ChecksConfigTest.test_load_bad_type
   PASS    5ms★  test_checks.ChecksConfigTest.test_load_valid_config
   PASS   19ms★  test_checks.ChecksExecuteTest.test_execute
   PASS  222ms   test_checks.ChecksExecuteTest.test_execute_concurrency
   PASS   18ms★  test_checks.ChecksExecuteTest.test_execute_failure
   PASS   15ms★  test_checks.ChecksExecuteTest.test_execute_timeout
   PASS   <1ms★  test_utils.UtilsTest.test_check_php_opening_tag
   PASS    3ms★  test_utils.UtilsTest.test_check_target_hosts
   PASS    1ms★  test_utils.UtilsTest.test_check_valid_json_file
   PASS   <1ms★  test_utils.UtilsTest.test_get_env_specific_filename
```

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D14363

src/unit/engine/NoseTestEngine.php

@@ -16,7 +16,8 @@ final class NoseTestEngine extends ArcanistUnitTestEngine {
   public function run() {
     if ($this->getRunAllTests()) {
       $root = $this->getWorkingCopy()->getProjectRoot();
-      return $this->runTests(array($root), './');
+      $all_tests = glob(Filesystem::resolvePath("$root/tests/**/test_*.py"));
+      return $this->runTests($all_tests, $root);
     }
 
     $paths = $this->getPaths();