Bug 1713145 - Also test content at root part of multipart/alternative emails for messages.getFull API. r=mkmelin

Differential Revision: https://phabricator.services.mozilla.com/D117621

--HG--
extra : amend_source : df4f460591df0c829e6cac9934bf71044dfdccf5

mail/components/extensions/test/xpcshell/messages/alternative.eml

@@ -7,6 +7,9 @@ Mime-Version: 1.0
 Content-Type: multipart/alternative;
 	boundary="=====================_714967308==_.ALT"
 
+This message is in MIME format.  The first part should be readable text,
+while the remaining parts are likely unreadable without MIME-aware tools.
+
 --=====================_714967308==_.ALT
 Content-Transfer-Encoding: quoted-printable
 

mail/components/extensions/test/xpcshell/messages/sample07.eml

@@ -7,6 +7,9 @@ Mime-Version: 1.0
 Content-Type: multipart/alternative;
 	boundary="=====================_714967308==_.ALT"
 
+This message is in MIME format.  The first part should be readable text,
+while the remaining parts are likely unreadable without MIME-aware tools.
+
 --=====================_714967308==_.ALT
 Content-Transfer-Encoding: quoted-printable