tests/blacklists/nix

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.

# Tests to be disabled when building and testing in the Nix sandbox.

# tests enforcing "/usr/bin/env" shebangs, which are patched for nix
test-run-tests.t
test-check-shbang.t

# doesn't like the extra setlocale warnings emitted by the nix bash wrappers
test-locale.t