Test coverage for native code WIP

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

tests/functional/meson.build

@@ -154,7 +154,7 @@ foreach script : functional_tests_scripts
     suite : 'installcheck',
     env : {
       'MESON_BUILD_ROOT': meson.project_build_root(),
-    },
+    } + coverage_test_env,
     # some tests take 15+ seconds even on an otherwise idle machine, on a loaded machine
     # this can easily drive them to failure. give them more time, 5min rather than 30sec
     timeout : 300,

tests/functional/repl_characterization/meson.build

@@ -36,6 +36,6 @@ test(
   env : {
     '_NIX_TEST_UNIT_DATA': meson.current_build_dir() / 'data',
     'MESON_BUILD_ROOT': meson.project_build_root(),
-  },
+  } + coverage_test_env,
   suite : 'installcheck',
 )