Feature: allow supressing some package init logics

[xhd2015]

When running unit test, some packages are annoying.Though we are not going to use it while testing, it still stops testing.

Example: vendor/github.com/lucas-clemente/quic-go/internal/handshake/unsafe.go

func init() {
	if !structsEqual(&tls.ConnectionState{}, &qtls.ConnectionState{}) {
		panic("qtls.ConnectionState not compatible with tls.ConnectionState")
	}
	if !structsEqual(&tls.ClientSessionState{}, &qtls.ClientSessionState{}) {
		panic("qtls.ClientSessionState not compatible with tls.ClientSessionState")
	}
}

I want to rewrite it into

func init() {
        if true {
            return
        }
	if !structsEqual(&tls.ConnectionState{}, &qtls.ConnectionState{}) {
		panic("qtls.ConnectionState not compatible with tls.ConnectionState")
	}
	if !structsEqual(&tls.ClientSessionState{}, &qtls.ClientSessionState{}) {
		panic("qtls.ClientSessionState not compatible with tls.ClientSessionState")
	}
}

[xhd2015]

A solution that does not require changing xgo, using shell to help patch and restore the source file:

#!/usr/bin/env bash

patch_file=vendor/github.com/lucas-clemente/quic-go/internal/handshake/unsafe.go
orig_content=$(cat "$patch_file")

function apply_patch_file {
    cat > "$patch_file" <<EOF
//go:build ignore
// +build ignore

$orig_content
EOF
}

function restore_patch_file {
  echo "$orig_content" > "$patch_file"
}

apply_patch_file
trap restore_patch_file EXIT

xgo test -mod=vendor -v ./...