tests: Add file docstring to test_*.py files that were missing it

While at it, reoder import lines to better match the usual order
of having 'import ...' lines first, and then 'from ... import ...'
lines.

tests/test_annotate.py

@@ -1,12 +1,14 @@
+# -*- coding: utf-8 -*-
+"""Test cases for 'src/diffannotator/annotate.py' module"""
 import copy
 import re
 from pathlib import Path
 from textwrap import dedent
 
-from pygments.lexers import CLexer
-from pygments.token import Token
 import pytest
 import unidiff
+from pygments.lexers import CLexer
+from pygments.token import Token
 
 from diffannotator.annotate import (split_multiline_lex_tokens, line_ends_idx,
                                     group_tokens_by_line, front_fill_gaps, deep_update,

tests/test_cli.py

@@ -1,3 +1,8 @@
+# -*- coding: utf-8 -*-
+"""Test cases for all typer-based command line (CLI) scripts
+
+https://typer.tiangolo.com/tutorial/testing/
+"""
 import subprocess
 from pathlib import Path
 

tests/test_gather_data.py

@@ -1,3 +1,5 @@
+# -*- coding: utf-8 -*-
+"""Test cases for 'src/diffannotator/gather_data.py' module"""
 from collections import Counter
 from pathlib import Path
 

tests/test_languages.py

@@ -1,3 +1,5 @@
+# -*- coding: utf-8 -*-
+"""Test cases for 'src/diffannotator/languages.py' module"""
 import logging
 import pytest
 from pytest import LogCaptureFixture

tests/test_lexer.py

@@ -1,3 +1,5 @@
+# -*- coding: utf-8 -*-
+"""Test cases for 'src/diffannotator/lexer.py' module"""
 from textwrap import dedent
 
 from pygments.lexer import Lexer as PygmentsLexer