Tests: pathlib used where appropriate

Also fixed various pylint errors and style issues.

test/test_node_application.py

@@ -1,6 +1,7 @@
 import re
 
 import pytest
+
 from unit.applications.lang.node import ApplicationNode
 from unit.utils import waitforfiles
 
@@ -149,11 +150,13 @@ def test_node_application_write_buffer():
 
     assert client.get()['body'] == 'buffer', 'write buffer'
 
+
 def test_node_application_write_array():
     client.load('write_array')
 
     assert client.get()['body'] == 'array', 'write array'
 
+
 def test_node_application_write_callback(temp_dir):
     client.load('write_callback')
 
@@ -303,11 +306,13 @@ def test_node_application_get_header_names():
         'x-header',
     ], 'get header names'
 
+
 def test_node_application_flush_headers():
     client.load('flush_headers')
 
     assert client.get()['headers']['X-Header'] == 'blah'
 
+
 def test_node_application_has_header():
     client.load('has_header')