Timeout browser suite loads - #2080

Fixes #2691

The browser platform gave up waiting for a browser to load a compiled
suite after the test timeout, or 30 seconds if that was longer. This
timeout was added in #2080, before `suite_load_timeout` existed, to get
better diagnostics for browser loads that never completed. The test
timeout has nothing to do with loading, and `suite_load_timeout` had no
effect on this limit, so the only way to give a slow browser more time
was to raise the timeout for every test.

Start a stopwatch in the load suite's body, right after the invoker
starts the load test's timeout, and expose the time left before that
timeout through a static `PlatformPlugin.remainingLoadTime` getter that
platforms can read during `load`. It reads a zone value set by the load
suite, so platforms need no new imports, and it is static so classes
implementing `PlatformPlugin` don't need to change. It is `null` when
loading can't time out because the suite load timeout is `none` or
timeouts are ignored. The value is only meaningful during `load`;
callbacks registered while loading run in the same zone and may see a
stale value later.

Have the browser platform give up 1 second before the remaining load
time runs out, so its error, which includes the browser output, wins
over the generic timeout on the load test. Retries in the loader share
the same budget. When there is no remaining load time, don't time out
the browser, matching how other platforms load suites without a suite
load timeout. Configuring `suite_load_timeout` restores a limit along
with the more detailed error.

Bump test_core to 0.6.22-wip for the new API and pin it from test.