test: fix tarantool process teardown

Test uses a popen module that starts tarantool process in background
mode. Tarantool process started in background mode forks a new process
and closes a parent, after that popen loses a PID of the started process
and `ph:kill()` and `ph:terminate()` doesn't work anymore. It leads to
non-terminated tarantool processes after running the test.

Patch fixes that by running `kill` using os.execute with a PID of
tarantool process written to a pid file.

Follows up tarantool#6128

NO_CHANGELOG=fix test
NO_DOC=fix test

test/app-luatest/gh_6128_background_mode_test.lua

@@ -53,12 +53,11 @@ pid_file='%s', background=true, work_dir='%s', log='%s',
     t.helpers.retrying({timeout = 2, delay = 0.01}, function(path)
         assert(fio.path.exists(path) == true)
     end, g.log_path)
+    g.pid = fio.open(g.pid_path):read()
 end)
 
 g.after_test("test_background_mode_box_cfg", function(cg)
-    cg.ph:terminate()
-    cg.ph:wait()
-    cg.ph:close()
+    os.execute("kill -9 " .. cg.pid)
     fio.unlink(cg.pid_path)
     fio.unlink(cg.log_path)
     os.remove("*.xlog")
@@ -94,12 +93,11 @@ g.before_test("test_background_mode_env_vars", function()
     t.helpers.retrying({timeout = 2, delay = 0.01}, function(path)
         assert(fio.path.exists(path) == true)
     end, g.log_path)
+    g.pid = fio.open(g.pid_path):read()
 end)
 
 g.after_test("test_background_mode_env_vars", function(cg)
-    cg.ph:terminate()
-    cg.ph:wait()
-    cg.ph:close()
+    os.execute("kill -9 " .. cg.pid)
     fio.unlink(cg.pid_path)
     fio.unlink(cg.log_path)
     os.remove("*.xlog")