Disallow non-existent paths in sdk package

Test behavior was changed a while ago so that tests by default ignore
non-existent source paths (unless they explicitly check for/rely on
them). Prior to that CheckSnapshot() could detect when files were
missing from the snapshot but it no longer can.

This change disallows non-existent source files in all the sdk tests
which means that they are disallowed when processing the snapshots as
they use the same preparers as were used to process the sources.

This caused a test failure which has been temporarily ignored and has
a TODO and bug associated with it.

Bug: 183184375
Test: m nothing
Change-Id: I969d8515d20ef5ae515f2b5f93d8ed4e4f8ede75