Add the MultiprocessExec test and its test

util/test/posix/close_multiple.h

 // Copyright 2014 The Crashpad Authors. All rights reserved.
 //
 // Licensed under the Apache License, Version 2.0 (the "License");
 // you may not use this file except in compliance with the License.
 // You may obtain a copy of the License at
 //
 //     http://www.apache.org/licenses/LICENSE-2.0
 //
 // Unless required by applicable law or agreed to in writing, software
 // distributed under the License is distributed on an "AS IS" BASIS,
@@ skipping 16 matching lines @@
 //! child process does not inherit file descriptors that it has no need for.
 //!
 //! Unlike the BSD function, this function may not close file descriptors
 //! immediately, but may instead mark them as close-on-exec. The actual behavior
 //! chosen is specific to the operating system. On Mac OS X, file descriptors
 //! are marked close-on-exec instead of being closed outright in order to avoid
 //! raising `EXC_GUARD` exceptions for guarded file descriptors that are
 //! protected against `close()`.
 //!
 //! \param[in] fd The lowest file descriptor to close or set as close-on-exec.
-void CloseMultipleNowOrOnExec(int fd);
+//! \param[in] preserve_fd A file descriptor to preserve and not close (or set
+//!     as close-on-exec), even if it is open and its value is greater than \a
+//!     fd. To not preserve any file descriptor, pass `-1` for this parameter.
+void CloseMultipleNowOrOnExec(int fd, int preserve_fd);
 
 }  // namespace crashpad
 
 #endif  // CRASHPAD_UTIL_TEST_POSIX_CLOSE_MULTIPLE_H_