[wpilib] PIDController: Recompute the error in AtSetpoint() (#2822)

This makes AtSetpoint() return false after the setpoint is changed with
SetSetpoint().

Closes #2821.

Co-authored-by: Prateek Machiraju <prateek.machiraju@gmail.com>

wpilibj/src/test/java/edu/wpi/first/wpilibj/controller/PIDToleranceTest.java

@@ -31,9 +31,17 @@ class PIDToleranceTest {
     try (var controller = new PIDController(0.05, 0.0, 0.0)) {
       controller.enableContinuousInput(-kRange / 2, kRange / 2);
 
+      assertTrue(controller.atSetpoint(),
+          "Error was not in tolerance when it should have been. Error was "
+          + controller.getPositionError());
+
       controller.setTolerance(kTolerance);
       controller.setSetpoint(kSetpoint);
 
+      assertFalse(controller.atSetpoint(),
+          "Error was in tolerance when it should not have been. Error was "
+          + controller.getPositionError());
+
       controller.calculate(0.0);
 
       assertFalse(controller.atSetpoint(),