Fix luacheck warnings in test/app

Part of #5454

Reviewed-by: Vladislav Shpilevoy <v.shpilevoy@tarantool.org>
Reviewed-by: Igor Munkin <imun@tarantool.org>

Co-authored-by: Vladislav Shpilevoy <v.shpilevoy@tarantool.org>
Co-authored-by: Igor Munkin <imun@tarantool.org>

.luacheckrc

@@ -32,7 +32,7 @@ exclude_files = {
     "build/**/*.lua",
     -- Third-party source code.
     "test-run/**/*.lua",
-    "test/app/**/*.lua",
+    "test/app/*.test.lua",
     "test/box/**/*.lua",
     "test/box-py/**/*.lua",
     "test/box-tap/**/*.lua",
@@ -74,3 +74,6 @@ files["test/app-tap/lua/require_mod.lua"] = {
 files["test/app-tap/string.test.lua"] = {
     globals = {"utf8"}
 }
+files["test/app/lua/fiber.lua"] = {
+    globals = {"box_fiber_run_test"}
+}