Migrator: Infer late for test variables

Fixes https://github.com/dart-lang/sdk/issues/38291

This change also involves moving the "package" where test sources are
written and analyzed; It used to be /home/test (or /project in some
places); and a pubspec.yaml was written (or a .packages file) which
specified the package's name was "test". However, this conflicts with
adding an entry for a mock copy of the actual _test_ package. A
.packages file cannot have two entries for a package named "test", so
the package where all test sources are written is changed to "tests"
(located at /home/tests).

Change-Id: I462b88a814931dc2d4f1e72d07e3daf64768a399
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/144994
Commit-Queue: Samuel Rawlins <srawlins@google.com>
Reviewed-by: Paul Berry <paulberry@google.com>