Basic test that we can use private binaries as public ones

  $ dune build
  Executing priv as priv
  PATH:
  	$TESTCASE_ROOT/_build/default/using-priv/.bin
  	$TESTCASE_ROOT/_build/install/default/bin
  Executing priv as priv-renamed
  PATH:
  	$TESTCASE_ROOT/_build/default/using-priv/.bin
  	$TESTCASE_ROOT/_build/install/default/bin
