Navigate to about:blank after loading a crashtest (#193)

This helps catch a class of delayed crashes that are currently difficult
to attribute to a specific test and so difficult to provide correct metadata
for.

Author: jgraham

rfcs/crash_test_navigate.md

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+# RFC 193: Navigate after Crash Tests
+
+## Summary
+
+Navigate to about:blank after successfully loading the test URL in
+crash tests.
+
+## Details
+
+Add a step to the end of crash tests where the browsing context that
+loaded the test is navigated to about:blank.
+
+This is because we have seen cases where the crash the test is trying
+to detect doesn't happen until a subsequent navigation. That kind of
+crash is hard to handle in web-platform-tests: if the test happens to
+be the last one in a group it may be missed entirely, and because we
+want to make it possible to import tests without blocking on fixing
+the bugs they reveal, it's important to be able to annotate a crash as
+related to a specific test so the expectation metadata can be set
+correctly; if the actual crash happens at the start of the following
+test this doesn't work because we've already recorded an incorrect
+PASS status for the previous test. It's also helpful for humans to see
+the test that's actually failing when debugging the problem.
+
+## Risks
+
+This will decrease the performance of crash tests. However since
+they're few in number and generally fast compared to other test types,
+this seems like an acceptable tradeoff.
+
+Since about:blank is a special URL it's possible that navigating to
+that is triggering different codepaths to navigating to an actual
+server-provided resource. We could fix this by navigating to a blank
+page on the server, but this would likely have even worse performance
+implications. We should consider this option if we find there are
+cases not caught by this approach that would be caught when navigating
+to a http URL.
+
+This won't cover all possible cases of delayed crashes; in general
+that's an impossible problem to solve. However it is an incremental
+improvement over the status quo.