Improve errors when LSP integration tests fail to spawn server

When the server fails to start up (writing to stderr) the tests currently would just hang waiting for responses to messages. This will just throw the message as an exception, failing the test with a more useful message, for example:

```
00:07 +0 -1: ServerTest | test_exit_afterShutdown [E]
  Analysis Server wrote to stderr:

  file:///Users/dantup/Dev/Google/dart-sdk/sdk/third_party/pkg/linter/lib/src/analyzer.dart:106:27: Error: The method 'registerDefault' isn't defined for the class 'Registry'.
   - 'Registry' is from 'package:analyzer/src/lint/registry.dart' ('file:///Users/dantup/Dev/Google/dart-sdk/sdk/pkg/analyzer/lib/src/lint/registry.dart').
  Try correcting the name to the name of an existing method, or defining a method named 'registerDefault'.
      Registry.ruleRegistry.registerDefault(lint);
                            ^^^^^^^^^^^^^^^
```

Change-Id: If64b68b17a0d379f8879891f0346704d4ad360e8

Remove unused imports

Change-Id: I745499f7b038c81320c127410967d939c7756e51
Reviewed-on: https://dart-review.googlesource.com/c/94222
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Commit-Queue: Danny Tuppeny <dantup@google.com>