[hal] Set HAL Notifier thread as RT by default (#3482)

This PR gives the Notifier HAL thread RT priority 40 in RobotBase after
HAL initialization and before the user code is run. This drastically
improves scheduling jitter for TimedRobot's AddPeriodic() functions (in
3512's experience).

It's too risky to set user code as RT because badly behaved code
will lock up the Rio (potentially requiring safe mode to recover).

This needs the user program to be setuid admin to succeed.

wpilibcIntegrationTests/src/main/native/cpp/PriorityTest.cpp

@@ -0,0 +1,68 @@
+// 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 <thread>
+
+#include <wpi/condition_variable.h>
+#include <wpi/mutex.h>
+
+#include "frc/Threads.h"
+#include "gtest/gtest.h"
+
+TEST(PriorityTest, SetThreadPriority) {
+  bool exited = false;
+  wpi::condition_variable cond;
+  wpi::mutex mutex;
+  std::thread thr{[&] {
+    std::unique_lock lock{mutex};
+    cond.wait(lock, [&] { return exited; });
+  }};
+
+  // Non-RT thread has priority of zero
+  bool isRealTime = false;
+  EXPECT_EQ(0, frc::GetThreadPriority(thr, &isRealTime));
+  EXPECT_FALSE(isRealTime);
+
+  // Set thread priority to 15
+  EXPECT_TRUE(frc::SetThreadPriority(thr, true, 15));
+
+  // Verify thread was given priority 15
+  EXPECT_EQ(15, frc::GetThreadPriority(thr, &isRealTime));
+  EXPECT_TRUE(isRealTime);
+
+  // Set thread back to non-RT
+  EXPECT_TRUE(frc::SetThreadPriority(thr, false, 0));
+
+  // Verify thread is now non-RT
+  EXPECT_EQ(0, frc::GetThreadPriority(thr, &isRealTime));
+  EXPECT_FALSE(isRealTime);
+
+  {
+    std::scoped_lock lock{mutex};
+    exited = true;
+  }
+  cond.notify_one();
+  thr.join();
+}
+
+TEST(PriorityTest, SetCurrentThreadPriority) {
+  // Non-RT thread has priority of zero
+  bool isRealTime = true;
+  EXPECT_EQ(0, frc::GetCurrentThreadPriority(&isRealTime));
+  EXPECT_FALSE(isRealTime);
+
+  // Set thread priority to 15
+  EXPECT_TRUE(frc::SetCurrentThreadPriority(true, 15));
+
+  // Verify thread was given priority 15
+  EXPECT_EQ(15, frc::GetCurrentThreadPriority(&isRealTime));
+  EXPECT_TRUE(isRealTime);
+
+  // Set thread back to non-RT
+  EXPECT_TRUE(frc::SetCurrentThreadPriority(false, 0));
+
+  // Verify thread is now non-RT
+  EXPECT_EQ(0, frc::GetCurrentThreadPriority(&isRealTime));
+  EXPECT_FALSE(isRealTime);
+}