test: make T450 independent of application/octet-stream interpretation

test: make T450 independent of application/octet-stream interpretation

The actual content type of `application/octet-stream` is up to content
type detection of the reader, and thus may not be stable across
implementations or versions. This showed up when

fd46fc19 ("emacs:  document/defcustom notmuch-multipart/alternative-discouraged", 2022-05-14)

introduced a test for omitting a part of type `text/html` because it
expected a part of type `application/octet-stream` to remain in place,
i.e. a part of "unstable type". In particular, tests with `fd46fc19`
would succeed on RHEL/EPEL but fail on all current Fedoras with

```
 FAIL   multipart/alternative hides html by default
	--- T450-emacs-show.16.notmuch-show-multipart-alternative	2022-05-26 15:34:42.100557244 +0000
	+++ T450-emacs-show.16.OUTPUT	2022-05-26 15:34:42.102557207 +0000
	@@ -24,7 +24,7 @@
	 uses 64 as the
	 buffer size.
	 [ text/html (hidden) ]
	-[ 0001-Deal-with-situation-where-sysconf-_SC_GETPW_R_SIZE_M.patch: application/octet-stream (as text/x-diff) ]
	+[ 0001-Deal-with-situation-where-sysconf-_SC_GETPW_R_SIZE_M.patch: application/octet-stream (as text/x-patch) ]
	 From e3bc4bbd7b9d0d086816ab5f8f2d6ffea1dd3ea4 Mon Sep 17 00:00:00 2001
	 From: Alexander Botero-Lowry <alex.boterolowry@gmail.com>
	 Date: Tue, 17 Nov 2009 11:30:39 -0800
```

due to the different type detected.

Fix this by giving that message a specicific type of `text/x-diff` in
the test corpus, and adjust all affected test outputs.

Signed-off-by: Michael J Gruber <git@grubix.eu>
Amended-by: db, fix some trailing whitespace