tests: add more tests for "hg shelve --delete"

It appears that the only tests for "hg shelve --delete" concern command
errors (e.g. incompatible command options). Adding some more to check
that non-existent names are handled and a success case.

Differential Revision: https://phab.mercurial-scm.org/D7460

tests/test-shelve.t

@@ -951,6 +951,16 Test shelve --keep
   +++ b/jungle
   @@ -0,0 +1,1 @@
   +babar
+
+Test shelve --delete
+
+  $ hg shelve --list
+  default         (*s ago)    changes to: create conflict (glob)
+  $ hg shelve --delete doesnotexist
+  abort: shelved change 'doesnotexist' not found
+  [255]
+  $ hg shelve --delete default
+
   $ cd ..
 
 Test visibility of in-memory changes inside transaction to external hook