Bug 1482344 - [raptor] Fix fetch tasks for native-engine mozharness_test based tasks, r=jmaher

This unbreaks some tier 3 raptor tasks. There are a few fixes rolled together here:
1) Stop overwriting the 'env' in mozharness_test.py's 'native-engine' implementation
2) Set the workdir to /home/cltbld (which makes sure the fetches are downloaded to there)
3) Download the fetches via mozharness in the 'raptor' script (since they don't use run-task anymore)

Depends on D3651

Differential Revision: https://phabricator.services.mozilla.com/D3652

--HG--
extra : moz-landing-system : lando

taskcluster/ci/test/raptor.yml

@@ -1,6 +1,7 @@
 job-defaults:
     max-run-time: 1800
     suite: raptor
+    workdir: /home/cltbld
     tier:
         by-test-platform:
             windows10-64-ccov/.*: 3

taskcluster/taskgraph/transforms/job/mozharness_test.py

@@ -337,7 +337,8 @@ def mozharness_test_on_native_engine(config, job, taskdesc):
     if test['max-run-time']:
         worker['max-run-time'] = test['max-run-time']
 
-    worker['env'] = env = {
+    env = worker.setdefault('env', {})
+    env.update({
         'GECKO_HEAD_REPOSITORY': config.params['head_repository'],
         'GECKO_HEAD_REV': config.params['head_rev'],
         'MOZHARNESS_CONFIG': ' '.join(mozharness['config']),
@@ -351,7 +352,7 @@ def mozharness_test_on_native_engine(config, job, taskdesc):
         "MOZ_HIDE_RESULTS_TABLE": '1',
         "MOZ_NODE_PATH": "/usr/local/bin/node",
         'MOZ_AUTOMATION': '1',
-    }
+    })
     # talos tests don't need Xvfb
     if is_talos:
         env['NEED_XVFB'] = 'false'

taskcluster/taskgraph/transforms/tests.py

@@ -146,6 +146,9 @@ test_description_schema = Schema({
         'test-platform',
         basestring),
 
+    # base work directory used to set up the task.
+    Optional('workdir'): basestring,
+
     # the name by which this test suite is addressed in try syntax; defaults to
     # the test-name.  This will translate to the `unittest_try_name` or
     # `talos_try_name` attribute.
@@ -1097,6 +1100,9 @@ def make_job_description(config, tests):
         run['using'] = 'mozharness-test'
         run['test'] = test
 
+        if 'workdir' in test:
+            run['workdir'] = test.pop('workdir')
+
         jobdesc['worker-type'] = test.pop('worker-type')
         if test.get('fetches'):
             jobdesc['fetches'] = test.pop('fetches')

testing/mozharness/mozharness/mozilla/testing/raptor.py

@@ -299,6 +299,8 @@ class Raptor(TestingMixin, MercurialScript, CodeCoverageMixin):
     # clobber defined in BaseScript
 
     def download_and_extract(self, extract_dirs=None, suite_categories=None):
+        if 'MOZ_FETCHES' in os.environ:
+            self.fetch_content()
         return super(Raptor, self).download_and_extract(
             suite_categories=['common', 'raptor']
         )