tests: fix up some lax escaping in test-template-basic.t These misfired escapes turn into hard errors in Python 3.7, and I'd really rather we not work around it. We should *probably* try and find a way to proactively warn users about invalid escape sequences. There's one more failure of this type in this file on Python 3.7, but I can't figure out the issue. It'll need to be corrected in a follow-up. Differential Revision: https://phab.mercurial-scm.org/D3843