Test coverage for native code WIP

Part of: https://git.lix.systems/lix-project/lix/issues/1186

tests/unit/meson.build

@@ -17,7 +17,7 @@ default_test_env = {
   'NIX_CONF_DIR': '/var/empty',
   # Prevent loading user configuration files in tests
   'NIX_USER_CONF_FILES': '',
-}
+} + coverage_test_env
 
 libutil_test_support_sources = files(
   'libutil-support/tests/cli-literate-parser.cc',
@@ -296,3 +296,11 @@ test(
   suite : 'check',
   protocol : 'gtest',
 )
+
+coverage_objects += [
+  libutil_tester,
+  libstore_tester,
+  libexpr_tester,
+  libcmd_tester,
+  libmain_tester,
+]