Mock: make --trap-stdlib the default when running `xgo test`

[xhd2015]

If we fixed the bug in https://github.com/xhd2015/xgo/issues/142, and verified that xgo test --trap-stdlib ./... ok in kubernetes, we will proceed on this as completing https://github.com/xhd2015/xgo/issues/103#issuecomment-2119618519.

But also allow xgo test --trap-stdlib=false to explicitly turn it off.