tests: 'hghave unix-permissions' for test-import.t

'hg import' of a patch with mode changes requires unix permissions support in
the file system.

tests/test-import.t

@@ -1,3 +1,5 b''
+  $ "$TESTDIR/hghave" unix-permissions || exit 80
+
   $ hg init a
   $ mkdir a/d1
   $ mkdir a/d1/d2