test-clone.t: add basic cases for destination ''

The case

  $ hg clone a ''

already aborted before 2649be11ab0b, whereas

  $ hg clone --pull a ''

tripped the fixed issue2528.

Both basic cases are expected to fail with an abort.

tests/test-clone.t

@@ -41,6 +41,12 b' Default operation:'
   checking files
   2 files, 11 changesets, 11 total revisions
 
+Invalid dest '' must abort:
+
+  $ hg clone . ''
+  abort: No such file or directory
+  [255]
+
 No update, with debug option:
 
   $ hg --debug clone -U . ../c
@@ -99,6 +105,12 b' Use --pull:'
   checking files
   2 files, 11 changesets, 11 total revisions
 
+Invalid dest '' with --pull must abort (issue2528):
+
+  $ hg clone --pull a ''
+  abort: No such file or directory
+  [255]
+
 Clone to '.':
 
   $ mkdir h