Add the MultiprocessExec test and its test

util/test/posix/close_multiple.h

Index: util/test/posix/close_multiple.h
diff --git a/util/test/posix/close_multiple.h b/util/test/posix/close_multiple.h
index f0199c16ef1191d754fd29d3fd541d96f7133f82..c2206bd057f33f0251e727d10feb4621f3097df9 100644
--- a/util/test/posix/close_multiple.h
+++ b/util/test/posix/close_multiple.h
@@ -34,7 +34,10 @@ namespace crashpad {
 //! 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