tests: add test ignore list for nix
Some tests cannot run properly in the nix sandbox.
This adds a list of tests to ignore when running in that context.
# 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