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/applications/lang/perl.py

@@ -6,19 +6,19 @@ class TestApplicationPerl(TestApplicationProto):
     application_type = "perl"
 
     def load(self, script, name='psgi.pl', **kwargs):
-        script_path = option.test_dir + '/perl/' + script
+        script_path = f'{option.test_dir}/perl/{script}'
 
         self._load_conf(
             {
-                "listeners": {"*:7080": {"pass": "applications/" + script}},
+                "listeners": {"*:7080": {"pass": f"applications/{script}"}},
                 "applications": {
                     script: {
                         "type": self.get_application_type(),
                         "processes": {"spare": 0},
                         "working_directory": script_path,
-                        "script": script_path + '/' + name,
+                        "script": f'{script_path}/{name}',
                     }
                 },
             },
-            **kwargs
+            **kwargs,
         )