test: use f-strings in feature_blocksdir.py

Author: fanquake

test/functional/feature_blocksdir.py

@@ -24,10 +24,10 @@ def run_test(self):
         initialize_datadir(self.options.tmpdir, 0, self.chain)
         self.log.info("Starting with nonexistent blocksdir ...")
         blocksdir_path = os.path.join(self.options.tmpdir, 'blocksdir')
-        self.nodes[0].assert_start_raises_init_error(["-blocksdir=" + blocksdir_path], 'Error: Specified blocks directory "{}" does not exist.'.format(blocksdir_path))
+        self.nodes[0].assert_start_raises_init_error([f"-blocksdir={blocksdir_path}"], f'Error: Specified blocks directory "{blocksdir_path}" does not exist.')
         os.mkdir(blocksdir_path)
         self.log.info("Starting with existing blocksdir ...")
-        self.start_node(0, ["-blocksdir=" + blocksdir_path])
+        self.start_node(0, [f"-blocksdir={blocksdir_path}"])
         self.log.info("mining blocks..")
         self.nodes[0].generatetoaddress(10, self.nodes[0].get_deterministic_priv_key().address)
         assert os.path.isfile(os.path.join(blocksdir_path, self.chain, "blocks", "blk00000.dat"))