Switch to bats over pytest

This removes python entirely from the stack.
Yet to run the tests on macOS, but they're buildable by way of nix
wrapping. Worst case scenario, you cannot run them from the devshell
but `nix run .#test-runner-<latest||stable>` should work okay.

tests/util.bash

@@ -0,0 +1,38 @@
+function _common_setup {
+  shopt -s globstar
+  bats_require_minimum_version 1.5.0
+  bats_load_library bats-support
+  bats_load_library bats-assert
+
+  TESTDIR=
+  TESTDIR=$(mktemp -d -t nix-direnv.XXXXXX)
+  export TESTDIR
+  export DIRENV_LOG_FORMAT="direnv: %s"
+
+  # Set up nix to be able to find your user's nix.conf if run locally
+  export NIX_USER_CONF_FILES="$HOME/.config/nix/nix.conf"
+
+  export HOME=$TESTDIR/home
+  unset XDG_DATA_HOME
+  unset XDG_CONFIG_HOME
+
+  cp "$BATS_TEST_DIRNAME"/testenv/* "$TESTDIR/"
+}
+
+function _common_teardown {
+  rm -rf "$TESTDIR"
+}
+
+function write_envrc {
+  echo "source $DIRENVRC" >"$TESTDIR/.envrc"
+  echo -e "\n$*" >>"$TESTDIR/.envrc"
+  direnv allow "$TESTDIR"
+}
+
+function run_in_direnv {
+  run --separate-stderr direnv exec "$TESTDIR" sh -c "$@"
+  assert_success
+  run direnv exec "$TESTDIR" sh -c "$@"
+  assert_success
+  assert_stderr -p "Renewed cache"
+}