tests: Port all test clients to the new test asserts

This commit gets rid of libc's abort() usage in the test suite using
test asserts instead.

Asserts run in the server as plugins aren't converted because they are
shared between server and client.

Co-authored-by: Daniel Stone <daniels@collabora.com>
Signed-off-by: Loïc Molinari <loic.molinari@collabora.com>