Stability script: normalize results format

[jugglinmike]

The "check stability" script parses test results into a List of tuples.
Later operations expect the fourth element to be a (possibly empty) List
of "messages". While this expectation is satisfied when the instability
is the result of a subtest's behavior, the parsing operation previously
violated the contract when interpreting test failures.

Update the parsing logic to provide an empty List in cases where a test
(rather than a subtest) is the source of instability.

[ghost]

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Firefox (nightly channel)

Testing web-platform-tests at revision 161fed7
Using browser at version BuildID 20170403100136; SourceStamp 38894655c89e68bcd8f45d31a0d3005f2c2b53db
Starting 10 test iterations
All results were stable

All results

2 tests ran

/service-workers/service-worker/fetch-event.https.html

Subtest	Results	Messages
	OK
Service Worker responds to fetch event with string	PASS
Service Worker responds to fetch event with blob body	PASS
Service Worker responds to fetch event with the referrer URL	PASS
Service Worker responds to fetch event with the referrer policy	FAIL	assert_unreached: unexpected rejection: NetworkError when attempting to fetch resource. Reached unreachable code
Service Worker responds to fetch event with an existing client id	PASS
Service Worker does not respond to fetch event	PASS
Service Worker responds to fetch event with null response body	PASS
Service Worker fetches other file in fetch event	PASS
Service Worker responds to fetch event with POST form	PASS
Multiple calls of respondWith must throw InvalidStateErrors	PASS
Service Worker event.respondWith must set the used flag	PASS
Service Worker should expose FetchEvent URL fragments.	PASS
Service Worker responds to fetch event with the correct cache types	PASS
Service Worker should intercept EventSource	PASS
Service Worker responds to fetch event with the correct integrity_metadata	PASS

/service-workers/service-worker/navigation-preload/redirect.https.html

Subtest	Results	Messages
	TIMEOUT
Navigation Preload redirect response.	TIMEOUT	Test timed out
Navigation Preload no-location redirect response.	NOTRUN
Navigation Preload no-location redirect response with body.	NOTRUN
Navigation Preload redirect to the same scope.	NOTRUN

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Chrome (unstable channel)

Testing web-platform-tests at revision 161fed7
Using browser at version 59.0.3053.3 dev
Starting 10 test iterations
All results were stable

All results

2 tests ran

/service-workers/service-worker/fetch-event.https.html

Subtest	Results	Messages
	OK
Service Worker responds to fetch event with string	PASS
Service Worker responds to fetch event with blob body	PASS
Service Worker responds to fetch event with the referrer URL	PASS
Service Worker responds to fetch event with the referrer policy	FAIL	assert_unreached: unexpected rejection: assert_equals: Service Worker should respond to fetch with the referrer URL when a member of RequestInit is present expected "Referrer: https://web-platform.test:8443/service-workers/service-worker/resources/simple.html?referrerPolicy\nReferrerPolicy: no-referrer-when-downgrade" but got "Referrer: https://web-platform.test:8443/service-workers/service-worker/resources/undefined\nReferrerPolicy: no-referrer-when-downgrade" Reached unreachable code
Service Worker responds to fetch event with an existing client id	PASS
Service Worker does not respond to fetch event	PASS
Service Worker responds to fetch event with null response body	PASS
Service Worker fetches other file in fetch event	PASS
Service Worker responds to fetch event with POST form	PASS
Multiple calls of respondWith must throw InvalidStateErrors	PASS
Service Worker event.respondWith must set the used flag	PASS
Service Worker should expose FetchEvent URL fragments.	FAIL	assert_unreached: unexpected rejection: assert_equals: Service worker should expose URL fragments in request. expected "Fragment Found :#/some/fragment" but got "Fragment Not Found" Reached unreachable code
Service Worker responds to fetch event with the correct cache types	FAIL	assert_unreached: unexpected rejection: assert_equals: expected "default" but got "" Reached unreachable code
Service Worker should intercept EventSource	FAIL	assert_unreached: unexpected rejection: assert_equals: EventSource should bypass the http cache. expected (string) "no-store" but got (undefined) undefined Reached unreachable code
Service Worker responds to fetch event with the correct integrity_metadata	PASS

/service-workers/service-worker/navigation-preload/redirect.https.html

Subtest	Results	Messages
	TIMEOUT
Navigation Preload redirect response.	TIMEOUT	Test timed out
Navigation Preload no-location redirect response.	NOTRUN
Navigation Preload no-location redirect response with body.	NOTRUN
Navigation Preload redirect to the same scope.	NOTRUN