test: test reception of multipart/alternative with text/calendar

In messages with text/plain, text/html and text/calendar parts
within a multipart/alternative, text/calendar part is currently ignored,
text/plain is displayed and HTML is available via HTML API.

test-data/message/calendar-alternative.eml

@@ -0,0 +1,30 @@
+From: Bob <bob@example.net>
+To: Alice <alice@example.org>
+Subject: Subject was here
+Date: Mon, 11 Aug 2025 10:15:52 +0000
+Message-ID: 
+	<DU2PR10MB7741CB7551F025C98CE1B0C0EE28A@DU2PR10MB7741.EURPRD10.PROD.OUTLOOK.COM>
+Content-Type: multipart/alternative;
+	boundary="_000_DU2PR10MB7741CB7551F025C98CE1B0C0EE28ADU2PR10MB7741EURP_"
+MIME-Version: 1.0
+
+--_000_DU2PR10MB7741CB7551F025C98CE1B0C0EE28ADU2PR10MB7741EURP_
+Content-Type: text/plain; charset="utf-8"
+
+Hello!
+--_000_DU2PR10MB7741CB7551F025C98CE1B0C0EE28ADU2PR10MB7741EURP_
+Content-Type: text/html; charset="utf-8"
+
+<b>Hello!</b>
+--_000_DU2PR10MB7741CB7551F025C98CE1B0C0EE28ADU2PR10MB7741EURP_
+Content-Type: text/calendar; charset="utf-8"; method=REQUEST
+
+BEGIN:VCALENDAR
+METHOD:REQUEST
+PRODID:Microsoft Exchange Server 2010
+VERSION:2.0
+...
+END:VCALENDAR
+
+--_000_DU2PR10MB7741CB7551F025C98CE1B0C0EE28ADU2PR10MB7741EURP_--
+