Allow handling multiple reruns in wptreport.json files

Previously when running with --rerun=N we added all the subtest results
to the previous test results and so got all N iterations of each
subtest result, but only one of each test result. Instead ensure that
top-level tests are added each time they are run, allowing multiple
test results.

Differential Revision: https://phabricator.services.mozilla.com/D47249

bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1584192
gecko-commit: 325b5278961fb32b8fd6d4dcc481c4dd7890f33c
gecko-integration-branch: autoland
gecko-reviewers: maja_zf

Author: jgraham

tools/wptrunner/wptrunner/formatters/wptreport.py

@@ -62,10 +62,10 @@ def suite_start(self, data):
         if 'run_info' in data:
             self.results['run_info'] = data['run_info']
         self.results['time_start'] = data['time']
+        self.results["results"] = []
 
     def suite_end(self, data):
         self.results['time_end'] = data['time']
-        self.results["results"] = []
         for test_name in self.raw_results:
             result = {"test": test_name}
             result.update(self.raw_results[test_name])
@@ -126,6 +126,11 @@ def test_end(self, data):
                 for item in data["extra"]["reftest_screenshots"]
                 if type(item) == dict
             }
+        test_name = data["test"]
+        result = {"test": data["test"]}
+        result.update(self.raw_results[test_name])
+        self.results["results"].append(result)
+        self.raw_results.pop(test_name)
 
     def assertion_count(self, data):
         test = self.find_or_create_test(data)