libweave: add missing tests to BUILD.gn

Many unit tests in libweave are not included in the BUILD.gn and hence
the tests are not actually run.
Add and fix them.

BUG=None
TEST=FEATURES=test emerge-hatch libweave

Change-Id: I9b5b7c49fd351b5aa4be2b629ecc30db1c75615e
Reviewed-on: https://chromium-review.googlesource.com/c/weave/libweave/+/3308950
Tested-by: Grace Cham <hscham@chromium.org>
Reviewed-by: Keigo Oka <oka@chromium.org>
Commit-Queue: Grace Cham <hscham@chromium.org>
4 files changed