test: replace Lua `assert`s with luatest `assert`s in luatest tests

Some luatest framework tests use Lua `assert`s, which are incomprehensible
when failed (the only information provided is 'assertion failed!'),
making debugging difficult: replace them with luatest `assert`s and their
context-specific varieties.

NO_CHANGELOG=<code health>
NO_DOC=<code health>

(cherry picked from commit 1d9645f7)