E2E tests in SF build: allow changing config files

In the installation for E2E testing (and ONLY in that installation, not
in the main build), we make htdocs/config group-writable so that the E2E
testing scripts can put the appropriate test configuration into place,
then remove it when they're finished. We also make sure that the scripts
putting that configuration into place are correctly looking in htdocs/
rather than in src/.