t/300-add.t: Fix warning

t/300-add.t ........ "my" variable $output masks earlier declaration in
same scope at t/300-add.t line 32.

t/300-add.t

@@ -29,7 +29,7 @@ A  a
 
 ", 'Adding a file works';
 
-my $output = `.././vcsh status --terse`;
+$output = `.././vcsh status --terse`;
 
 ok $output eq "test1:
 A  a