[wpiutil] DataLog: Ensure file is written on shutdown (#6087)

Previously the thread could end without the file being written.

wpiutil/src/test/native/cpp/DataLogTest.cpp

@@ -0,0 +1,18 @@
+// Copyright (c) FIRST and other WPILib contributors.
+// Open Source Software; you can modify and/or share it under the terms of
+// the WPILib BSD license file in the root directory of this project.
+
+#include <gtest/gtest.h>
+
+#include "wpi/DataLog.h"
+
+TEST(DataLogTest, SimpleInt) {
+  std::vector<uint8_t> data;
+  {
+    wpi::log::DataLog log{
+        [&](auto out) { data.insert(data.end(), out.begin(), out.end()); }};
+    int entry = log.Start("test", "int64");
+    log.AppendInteger(entry, 1, 0);
+  }
+  ASSERT_EQ(data.size(), 66u);
+}