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/node.py

@@ -13,16 +13,16 @@ class TestApplicationNode(TestApplicationProto):
     def prepare_env(self, script):
         # copy application
         shutil.copytree(
-            option.test_dir + '/node/' + script, option.temp_dir + '/node'
+            f'{option.test_dir}/node/{script}', f'{option.temp_dir}/node'
         )
 
         # copy modules
         shutil.copytree(
-            option.current_dir + '/node/node_modules',
-            option.temp_dir + '/node/node_modules',
+            f'{option.current_dir}/node/node_modules',
+            f'{option.temp_dir}/node/node_modules',
         )
 
-        public_dir(option.temp_dir + '/node')
+        public_dir(f'{option.temp_dir}/node')
 
     def load(self, script, name='app.js', **kwargs):
         self.prepare_env(script)
@@ -43,17 +43,17 @@ class TestApplicationNode(TestApplicationProto):
         self._load_conf(
             {
                 "listeners": {
-                    "*:7080": {"pass": "applications/" + quote(script, '')}
+                    "*:7080": {"pass": f"applications/{quote(script, '')}"}
                 },
                 "applications": {
                     script: {
                         "type": "external",
                         "processes": {"spare": 0},
-                        "working_directory": option.temp_dir + '/node',
+                        "working_directory": f'{option.temp_dir}/node',
                         "executable": '/usr/bin/env',
                         "arguments": arguments,
                     }
                 },
             },
-            **kwargs
+            **kwargs,
         )