Fix a race in an expanded reporter retry test - #2810

The test "retry: does not report an attempt for errors after completion"
flaked on master: the expected `+1: second test ...` progress line was
missing, and the output went straight to `+0 -1: second test ...`.

The first test scheduled its error with `Future(() { ... })`, so the test
isolate raised it right after the test completed, without waiting for the
runner. The runner starts the next test one event-loop turn after it
receives the `complete` message. If the error message was already queued
by then, the first test moved to failed before the second test started,
and the reporter printed different progress lines. Both orders are correct
reporter output; the test's expectation relied on the race.

Have the second test trigger the first test's error, using the same
`Completer` pattern as `retry_test.dart`. The error can't exist until the
second test is running in the isolate, after the runner has printed its
progress line, and the second test waits a turn so the error arrives before
it completes.

Locally, the old fixture produced the CI output in 1 of 40 runs. The new
fixture produced identical output in 60 of 60 runs.