add test for "compare_modules" function

tests/_data/imports.txt

@@ -0,0 +1,3 @@
+pandas==2.0.0
+numpy>=1.2.3
+torch<4.0.0

tests/test_pipreqs.py

@@ -25,8 +25,12 @@ def setUp(self):
             'after_method_is_valid_even_if_not_pep8'
             ]
         self.modules2 = ['beautifulsoup4']
+        self.parsed_packages = [{'name': 'pandas', 'version': '2.0.0'}, {'name': 'numpy', 'version': '1.2.3'}, {'name': 'torch', 'version': '4.0.0'}]
+
         self.local = ["docopt", "requests", "nose", 'pyflakes']
         self.project = os.path.join(os.path.dirname(__file__), "_data")
+        self.empty_filepath = os.path.join(self.project, "empty.txt")
+        self.imports_filepath = os.path.join(self.project, "imports.txt")
         self.project_clean = os.path.join(
             os.path.dirname(__file__),
             "_data_clean"
@@ -322,6 +326,23 @@ def test_clean_with_imports_to_clean(self):
             data = f.read().lower()
             self.assertTrue(cleaned_module not in data)
 
+    def test_compare_modules(self):
+        test_cases = [
+            ( self.empty_filepath, [], set() ), # both empty
+            ( self.empty_filepath, self.parsed_packages, set() ), # only file empty
+            ( self.imports_filepath, [], set(package["name"] for package in self.parsed_packages) ), # only imports empty
+            ( self.imports_filepath, self.parsed_packages, set() ), # no difference
+            ( self.imports_filepath, self.parsed_packages[1:], set([self.parsed_packages[0]["name"]]) ) # common case
+        ]
+
+        for test_case in test_cases:
+            with self.subTest(test_case):
+                filename, imports, expected_modules_not_imported = test_case
+
+                modules_not_imported = pipreqs.compare_modules(filename, imports)
+
+                self.assertSetEqual(modules_not_imported, expected_modules_not_imported)
+
     def tearDown(self):
         """
         Remove requiremnts.txt files that were written