unittests: extend machine file override tests for CFLAGS (and friends)

They behave slightly differently than other env vars, so have a test for
them as well.

run_unittests.py

@@ -9214,13 +9214,15 @@ class CrossFileTests(BasePlatformTests):
 
     def test_builtin_options_conf_overrides_env(self):
         testcase = os.path.join(self.common_test_dir, '2 cpp')
-        config = self.helper_create_cross_file({'built-in options': {'pkg_config_path': '/native'}})
-        cross = self.helper_create_cross_file({'built-in options': {'pkg_config_path': '/cross'}})
+        config = self.helper_create_cross_file({'built-in options': {'pkg_config_path': '/native', 'cpp_args': '-DFILE'}})
+        cross = self.helper_create_cross_file({'built-in options': {'pkg_config_path': '/cross', 'cpp_args': '-DFILE'}})
 
         self.init(testcase, extra_args=['--native-file', config, '--cross-file', cross],
-                  override_envvars={'PKG_CONFIG_PATH': '/bar', 'PKG_CONFIG_PATH_FOR_BUILD': '/dir'})
+                  override_envvars={'PKG_CONFIG_PATH': '/bar', 'PKG_CONFIG_PATH_FOR_BUILD': '/dir',
+                                    'CXXFLAGS': '-DENV', 'CXXFLAGS_FOR_BUILD': '-DENV'})
         configuration = self.introspect('--buildoptions')
         found = 0
+        expected = 4
         for each in configuration:
             if each['name'] == 'pkg_config_path':
                 self.assertEqual(each['value'], ['/cross'])
@@ -9228,9 +9230,12 @@ class CrossFileTests(BasePlatformTests):
             elif each['name'] == 'build.pkg_config_path':
                 self.assertEqual(each['value'], ['/native'])
                 found += 1
-            if found == 2:
+            elif each['name'].endswith('cpp_args'):
+                self.assertEqual(each['value'], ['-DFILE'])
+                found += 1
+            if found == expected:
                 break
-        self.assertEqual(found, 2, 'Did not find all sections.')
+        self.assertEqual(found, expected, 'Did not find all sections.')
 
 
 class TAPParserTests(unittest.TestCase):