Update bp2build tests to use stubbed defs

bp2build tests should not use bp2build_available:false test deps,
because, with the "allowlist v2" feature, this will cause these modules
to stop being converted with bp2build. Instead, such test deps should be
"stubbed", which effectively treats them like targets in pre-existing
BUILD files on the filesystem.

Bug: 301312260
Test: m nothing
Change-Id: Ib1ce7c87cd1e98568fdaf2a52a893dafa74ab327