test suite: Generate message filenames from count, not Message-Id.

We're about to add a test with an excessively long message-id, (512
characters or so). This exceeds filename length limits, so just always
the simple counter to generate the filenames, (which we were doing for
messages with non-custom IDs anyway).

test/notmuch-test

@@ -85,12 +85,12 @@ generate_message ()
     local -A template="($@)"
     local additional_headers
 
-    if [ -z "${template[id]}" ]; then
     gen_msg_cnt=$((gen_msg_cnt + 1))
     gen_msg_name=msg-$(printf "%03d" $gen_msg_cnt)
+
+    if [ -z "${template[id]}" ]; then
 	gen_msg_id="${gen_msg_name}@notmuch-test-suite"
     else
-	gen_msg_name="msg-${template[id]}"
 	gen_msg_id="${template[id]}"
     fi