Tests: switched to using f-strings.

Previously, it was necessary to support older versions of Python for
compatibility.  F-strings were released in Python 3.6.  Python 3.5 was
marked as unsupported by the end of 2020, so now it's possible to start
using f-strings safely for better readability and performance.

test/unit/check/chroot.py

@@ -12,7 +12,7 @@ def check_chroot():
     resp = http.put(
         url='/config',
         sock_type='unix',
-        addr=option.temp_dir + '/control.unit.sock',
+        addr=f'{option.temp_dir}/control.unit.sock',
         body=json.dumps(
             {
                 "listeners": {"*:7080": {"pass": "routes"}},