Automatically select main isolate instead of the test runner isolate when connecting to test runs

I don't know if this is a DevTools issue or something else, but I'm also not sure how to determine which.

When running tests, CPU profiling never seems to collect any data. For example given a sample project with a `bin/main.dart` that calls a function in `lib` that blocks for 5s, and a test that does the same:

## bin/main.dart

```dart
import 'package:cpu_profile/cpu_profile.dart' as cpu_profile;

void main(List arguments) {
print('starting'); // breakpoint
cpu_profile.calculate();
print('done!'); // breakpoint
}
```

## lib/cpu_profile.dart

```dart
int calculate() {
var start = DateTime.now();
while (start.difference(DateTime.now()).inSeconds.abs() < 5) {}
return 6 * 7;
}
```

## test/cpu_profile_test.dart

```dart
import 'package:cpu_profile/cpu_profile.dart' as cpu_profile;
import 'package:test/test.dart';

void main() {
test('calculate', () {
print('starting'); // breakpoint
cpu_profile.calculate();
print('done!'); // breakpoint
});
}
```

For `bin/main.dart`:

- Open `bin/main.dart`
- Add breakpoints on the marked lines
- Run in debug mode and hit the first breakpoint
- Open DevTools CPU profile, enable profiling, and start recording
- Resume execution and wait for it to hit the second breakpoint
- Stop CPU profiling and observe the results

Perform the same steps but using the test instead, and the profiler doesn't show any data:

Reproduce the issue with bin/main.dart and test/cpu_profile_test.dart, following the breakpoint and CPU profiling steps in the report. Start by tracing how DevTools connects to a test run and selects an isolate; done means profiling the test run captures the work in cpu_profile.calculate() and shows data like the standalone program.

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