test: fix several more tests after the luatest bump

The test gh_10088 was committed in parallel with the luatest bump and
thus slipped from the post-bump tests fixup in commit cfd4bf46
("test: adapt tests to the new luatest version"). Fix it now.

Also tests gh_6539 and gh_7231 queried `box.cfg.log` wrongly, but this
didn't make them fail, they just stopped testing what they were supposed
to. Fix them as well

NO_CHANGELOG=test
NO_TEST=test
NO_DOC=test

(cherry picked from commit 2a18de391895d8ec7a39e3d3dcee659fe79f7bc9)