vfstests: move functional tests from mountlib and make them work with VFS

The tests are now run for the mount commands and for the plain VFS.

This makes the tests much easier to debug when running with a VFS than
through a mount.

vfs/vfstest/write_non_unix.go

@@ -0,0 +1,25 @@
+// +build !linux,!darwin,!freebsd
+
+package vfstest
+
+import (
+	"runtime"
+	"testing"
+
+	"golang.org/x/sys/windows"
+)
+
+// TestWriteFileDoubleClose tests double close on write
+func TestWriteFileDoubleClose(t *testing.T) {
+	t.Skip("not supported on " + runtime.GOOS)
+}
+
+// writeTestDup performs the platform-specific implementation of the dup() syscall
+func writeTestDup(oldfd uintptr) (uintptr, error) {
+	p, err := windows.GetCurrentProcess()
+	if err != nil {
+		return 0, err
+	}
+	var h windows.Handle
+	return uintptr(h), windows.DuplicateHandle(p, windows.Handle(oldfd), p, &h, 0, true, windows.DUPLICATE_SAME_ACCESS)
+}