[infra] Let tool/test.py fail if the configuration supplied with -n does not exist
[infra] Let tool/test.py fail if the configuration supplied with -n does not exist

This will result in:

    tools/test.py -n foobar
    The named configuration "foobar" does not exist. The following configurations are available:
      * analyzer-asserts-linux
      * analyzer-asserts-mac
      * analyzer-asserts-win
      * ...


Change-Id: I2fadc2ab3ae6f2b34dd373d84365d44ef27c7b8b
Reviewed-on: https://dart-review.googlesource.com/c/87325
Reviewed-by: Daco Harkes <930cbae0ca77f1954c53e32620d89cfcca318f00@google.com>
Reviewed-by: Alexander Thomas <29642742b6693024c89de8232f2e2542cf7eedf7@google.com>
Commit-Queue: Martin Kustermann <a35dd5b79fac84b2bb3b026a16b8becc55af5961@google.com>

dart-archive/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dart-lang/sdk,dartino/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dart-lang/sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-lang/sdk