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)

test/box-luatest/gh_6539_log_user_space_empty_or_nil_select_test.lua

@@ -115,7 +115,9 @@ for _, eng in pairs{'memtx', 'vinyl'} do
                          'set'):format(eng))
         end
 
-        local log_file = g.server:eval('return box.cfg.log')
+        local log_file = g.server:exec(function()
+            return rawget(_G, 'box_cfg_log_file') or box.cfg.log
+        end)
         for _, call_fmt in pairs(dangerous_call_fmts) do
             local call = call_fmt:format(space)
             g.server:eval(call)

test/box-luatest/gh_7231_memtx_hash_idx_gt_iter_deprecation_test.lua

@@ -31,7 +31,9 @@ g.test_memtx_hash_idx_iter_gt_deprecation = function(cg)
                                 "not be used. It will be removed in a " ..
                                 "future Tarantool release."
     t.assert_is_not(cg.server:grep_log(deprecation_warning, 256), nil)
-    local log_file = g.server:exec(function() return box.cfg.log end)
+    local log_file = g.server:exec(function()
+        return rawget(_G, 'box_cfg_log_file') or box.cfg.log
+    end)
     fio.truncate(log_file)
     cg.server:exec(function()
         box.space.s:select({}, {iterator = 'GT'})