misc/cgo/testcshared: test -buildmode=c-shared

Followed the same test pattern in misc/cgo/testcarchive.

Change-Id: I2f863b5c24a28f0b38b0128ed3e8a92c17fb5b9f
Reviewed-on: https://go-review.googlesource.com/8985
Reviewed-by: David Crawshaw <crawshaw@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

misc/cgo/testcshared/src/libgo/libgo.go

@@ -0,0 +1,47 @@
+// Copyright 2015 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+import (
+	_ "p"
+	"syscall"
+	"time"
+)
+
+import "C"
+
+var initCh = make(chan int, 1)
+var ranMain bool
+
+func init() {
+	// emulate an exceedingly slow package initialization function
+	time.Sleep(100 * time.Millisecond)
+	initCh <- 42
+}
+
+func main() {
+	ranMain = true
+	panic("FAIL: main ran")
+}
+
+//export DidInitRun
+func DidInitRun() bool {
+	select {
+	case x := <-initCh:
+		if x != 42 {
+			// Just in case initCh was not correctly made.
+			println("want init value of 42, got: ", x)
+			syscall.Exit(2)
+		}
+		return true
+	default:
+		return false
+	}
+}
+
+//export DidMainRun
+func DidMainRun() bool {
+	return ranMain
+}