Move package bundle functionality into SummaryBaseTestStrategy.

The methods addBundle and createPackageBundle were previously in the
derived class SummaryLinkerTestStrategy, but there's no reason they
have to be there; they don't rely on any functionality that's specific
to linker tests.  Moving them will allow additional summary tests to
make use of package bundles.

Change-Id: I988147c76e6dcfef5ef0f000d66b1639b4b3ecf7
Reviewed-on: https://dart-review.googlesource.com/c/79280
Auto-Submit: Paul Berry <paulberry@google.com>
Commit-Queue: Konstantin Shcheglov <scheglov@google.com>
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>