test/emacs: globally force the html renderer to html2text

Previously we did this for a single test, but some other proposed
tests ( id:1398105468-14317-3-git-send-email-amdragon@mit.edu ) show
similar breakage when switching renderers.

test/T310-emacs.sh

@@ -483,7 +483,7 @@ test_expect_equal_file OUTPUT EXPECTED
 test_begin_subtest "Reply within emacs to an html-only message"
 add_message '[content-type]="text/html"' \
 	    '[body]="Hi,<br />This is an <b>HTML</b> test message.<br /><br />OK?"'
-test_emacs "(let ((message-hidden-headers '()) (mm-text-html-renderer 'html2text))
+test_emacs "(let ((message-hidden-headers '()))
 	    (notmuch-show \"id:${gen_msg_id}\")
 	    (notmuch-show-reply)
 	    (test-output))"

test/test-lib.el

@@ -181,3 +181,8 @@ nothing."
 ;; suite
 (setq notmuch-tag-deleted-formats
       '((".*" nil)))
+
+;; force a common html renderer, to avoid test variations between
+;; environments
+
+(setq mm-text-html-renderer 'html2text)