Change test script to prefer a named configuration when possible.
Change test script to prefer a named configuration when possible.

The VM requires some special options that are only provided by the test
matrix when running using a named config, so try to use a named
configuration when possible.

Change-Id: Ie725cbcfea5b02a35885b0601dc0c78f81e3917e
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/142940
Commit-Queue: Bob Nystrom <afede3a9ebedb865bee78398feff6fbe97068f17@google.com>
Auto-Submit: Bob Nystrom <afede3a9ebedb865bee78398feff6fbe97068f17@google.com>
Reviewed-by: Nicholas Shahan <03847abd2d8aadfe859ecb98095a31f9f515dc47@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk