The existing code for the `browser_aboutthirdparty.js` test attempts to
load a DLL indirectly by means of a Rube Goldberg process involving a
directly-instantiated file-picker. This appears to be the source of the
severe flakiness of this test.
This approach was reasonable when the file-picker was the primary source
of deleterious third-party DLLs; but now that we generally open file-
pickers out-of-process, there isn't as much of a reason to test that
particular injection mechanism. Let's just load the test DLL directly,
instead.
This patch has two caveats:
* The test fails _consistently_, rather than flakily, on ASAN builds.
The reason for that remains to be investigated, but probably belongs
in a separate bug; for now, we simply leave it disabled.
* The test expects the DLL to be a shell extension, and fails with
complaints if it's not. There's no other reason to have any of the
shell-extension-registration machinery in there, though, so it can
(and probably should) be removed, and the test rewritten accordingly.
In the interests of swiftly deflaking it, though, this is also being
left for a separate bug.
Outside of test-only code, no functional changes.
Differential Revision: https://phabricator.services.mozilla.com/D231462
aec3a47a80