Add --skip-soong-tests argument to soong_ui and use it in multiproduct_kati

There is no need for multiproduct_kati to run the tests for
every product, they don't vary by product config. --skip-soong-tests
can also be used for local development to run soong_build even if
the tests don't pass.

Bug: 156428456
Test: m --skip-soong-tests nothing
Change-Id: I9c00e3d1b6e51d17bb290339c3f124d4d1c9e69f