test: preserve the existing module policy config for testrepohg resuse The test runner is, righfully, setting HGMODULEPOLICY to match the flavor requests to run the tests. However this mess with the `testrepohg` ability to run properly, as the newly set policy might be incompatible with its installation, or its ability to read the test repo. So we preserve the initial value in a dedicated variable and use it in the `testrepohg` helper.