No longer copy functional tests to the build dir

This should make `_NIX_TEST_ACCEPT=1` work again, fixing #11369.

Progress on #2503

mk/common-test.sh

@@ -9,13 +9,21 @@ test_name=$(echo -n "${test?must be defined by caller (test runner)}" | sed \
     -e "s|\.sh$||" \
     )
 
+# Layer violation, but I am not inclined to care too much, as this code
+# is about to be deleted.
+src_dir=$(realpath tests/functional)
+
 # shellcheck disable=SC2016
 TESTS_ENVIRONMENT=(
     "TEST_NAME=$test_name"
     'NIX_REMOTE='
     'PS4=+(${BASH_SOURCE[0]-$0}:$LINENO) '
+    "_NIX_TEST_SOURCE_DIR=${src_dir}"
+    "_NIX_TEST_BUILD_DIR=${src_dir}"
 )
 
+unset src_dir
+
 read -r -a bash <<< "${BASH:-/usr/bin/env bash}"
 
 run () {