t/op/exec.t: Skip a test under -DUSE_SHELL_ALWAYS
t/lib/warnings/9uninit: Skip two tests under -DUSE_SHELL_ALWAYS
t/io/pipe.t: Skip test under -DUSE_SHELL_ALWAYS
ext/IPC-Open3/t/IPC-Open3.t: TODO a test under -DUSE_SHELL_ALWAYS