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/node.py

@@ -3,7 +3,7 @@ import subprocess
 
 
 def check_node(current_dir):
-    if not os.path.exists(current_dir + '/node/node_modules'):
+    if not os.path.exists(f'{current_dir}/node/node_modules'):
         return None
 
     try: