test: normalize only message filenames in show json

notmuch_json_show_sanitize replaced "filename" field values even in part
structures, where the value is predictable.  Make it only normalize the
filename value if it is an absolute path (begins with slash), which is
true of the Maildir filenames that were intended to be normalized away.

test/multipart

@@ -630,7 +630,7 @@ cat <<EOF >EXPECTED
  "content": "This is an embedded message, with a multipart/alternative part.\n"}]}]}]},
  {"id": 7,
  "content-type": "text/plain",
- "filename": "YYYYY",
+ "filename": "attachment",
  "content": "This is a text attachment.\n"},
  {"id": 8,
  "content-type": "text/plain",

test/test-lib.sh

@@ -600,7 +600,7 @@ notmuch_json_show_sanitize ()
 {
     sed \
 	-e 's|"id": "[^"]*",|"id": "XXXXX",|g' \
-	-e 's|"filename": "[^"]*",|"filename": "YYYYY",|g'
+	-e 's|"filename": "/[^"]*",|"filename": "YYYYY",|g'
 }
 
 # End of notmuch helper functions