[tree] unify integration tests into a single unit, except for diff

Author: jgarzik

tree/tests/link-tests.rs tree/tests/link/mod.rstree/tests/link-tests.rs renamed to tree/tests/link/mod.rs

@@ -8,3 +8,11 @@
 //
 
 mod cp;
+mod link;
+mod ls;
+mod mkdir;
+mod mv;
+mod readlink;
+mod rm;
+mod rmdir;
+mod unlink;