Add a testcase for optional output discovery

This test correctly fails with:
```
galaxy.jobs.runners.local ERROR 2018-03-02 17:55:32,396 Job wrapper finish method failed
Traceback (most recent call last):
  File "/Users/mvandenb/src/galaxy/lib/galaxy/jobs/runners/local.py", line 152, in queue_job
    self._finish_or_resubmit_job(job_state, stdout, stderr, exit_code)
  File "/Users/mvandenb/src/galaxy/lib/galaxy/jobs/runners/__init__.py", line 432, in _finish_or_resubmit_job
    job_state.job_wrapper.finish(stdout, stderr, exit_code, check_output_detected_state=check_output_detected_state)
  File "/Users/mvandenb/src/galaxy/lib/galaxy/jobs/__init__.py", line 1381, in finish
    'primary': self.tool.collect_primary_datasets(out_data, self.get_tool_provided_job_metadata(), tool_working_directory, input_ext, input_dbkey)
  File "/Users/mvandenb/src/galaxy/lib/galaxy/tools/__init__.py", line 1614, in collect_primary_datasets
    return output_collect.collect_primary_datasets(self, output, tool_provided_metadata, job_working_directory, input_ext, input_dbkey=input_dbkey)
  File "/Users/mvandenb/src/galaxy/lib/galaxy/tools/parameters/output_collect.py", line 372, in collect_primary_datasets
    for discovered_file in discover_files(name, tool_provided_metadata, dataset_collectors, job_working_directory, outdata):
  File "/Users/mvandenb/src/galaxy/lib/galaxy/tools/parameters/output_collect.py", line 490, in discover_files
    for (match, collector) in walk_over_file_collectors(extra_file_collectors, job_working_directory, matchable):
  File "/Users/mvandenb/src/galaxy/lib/galaxy/tools/parameters/output_collect.py", line 507, in walk_over_file_collectors
    for match in walk_over_extra_files(extra_file_collector.directory, extra_file_collector, job_working_directory, matchable):
  File "/Users/mvandenb/src/galaxy/lib/galaxy/tools/parameters/output_collect.py", line 519, in walk_over_extra_files
    for filename in os.listdir(directory):
OSError: [Errno 2] No such file or directory: '/private/var/folders/df/6xqpqpcd7h73b6jpx9t6cwhw0000gn/T/tmp5Tz0m9/tmpfUHHl6/tmpVBcnfI/database/job_working_directory_LJioiW/000/2/working/i_do_not_exist'
```

test/functional/tools/multi_output_recurse.xml

@@ -1,4 +1,4 @@
-<tool id="multi_output_recurse" name="multi_output_recurse" version="0.1.0">
+<tool id="multi_output_recurse" name="multi_output_recurse" version="0.1.0" profile="18.01">
   <command>
     echo "Hello" > $report;
     mkdir subdir1;
@@ -23,6 +23,9 @@
     <data auto_format="true" name="report">
       <discover_datasets pattern="__designation__" directory="." visible="true" recurse="true" />
     </data>
+    <data auto_format="true" name="report_optional">
+      <discover_datasets pattern="__designation__" directory="i_do_not_exist" visible="true" recurse="true" optional="true" />
+    </data>
   </outputs>
   <tests>
     <test>