Add luatest and checks as a submodule

Provide luatest repository as a submodule for execute luatest tests
inside workflows. Checks is a dependency for a luatest.

Needed for: #304

Author: VitaliyaIoffe

.gitmodules

@@ -4,3 +4,9 @@
 [submodule "lib/tarantool-python"]
 	path = lib/tarantool-python
 	url = https://github.com/tarantool/tarantool-python.git
+[submodule "luatest"]
+	path = luatest
+	url = https://github.com/tarantool/luatest
+[submodule "checks"]
+	path = checks
+	url = https://github.com/tarantool/checks.git

.luacheckrc

@@ -27,5 +27,7 @@ include_files = {
 exclude_files = {
     "lib/tarantool-python",
     "test/test-tarantool/*.test.lua",
-    ".rocks/**"
+    ".rocks/**",
+    "luatest/**",
+    "checks/**",
 }