Make writes thread-safe during local tests by opening files with O_APPEND.

--
MOS_MIGRATED_REVID=101572272

src/main/tools/process-tools.c

diff --git a/src/main/tools/process-tools.c b/src/main/tools/process-tools.c
index 48441e8..e80131b 100644
--- a/src/main/tools/process-tools.c
+++ b/src/main/tools/process-tools.c
@@ -50,7 +50,8 @@
 void Redirect(const char *target_path, int fd, const char *name) {
   if (target_path != NULL && strcmp(target_path, "-") != 0) {
     int fd_out;
-    CHECK_CALL(fd_out = open(target_path, O_WRONLY | O_CREAT | O_TRUNC, 0666),
+    const int flags = O_WRONLY | O_CREAT | O_TRUNC | O_APPEND;
+    CHECK_CALL(fd_out = open(target_path, flags, 0666),
                "Could not open %s for redirection of %s", target_path, name);
     CHECK_CALL(dup2(fd_out, fd));
     CHECK_CALL(close(fd_out));