These tests verify that the various components of LLCPP work together to deliver a particular feature. For example, handle_rights_test.cc tests that the client and server dispatchers verify handle type and down-scope handle rights. flexible_test.cc tests that LLCPP over-allocates memory for flexible types, in anticipation of receiving unknown fields pushing the message over its statically computed size limit.
It's good to have a few end-to-end tests verifying the behavior at a high level, but we should prefer writing tests with a more granular coverage, and over a smaller interface, such that the resulting tests are easier to setup and interpret, acknowledging the gap in the status quo.