[wpimath] Move SlewRateLimiter from wpilib to wpimath (#3399)

Timer was replaced with wpi::Now() to avoid a dependency on other wpilib
classes.

wpiutil/src/test/java/edu/wpi/first/wpiutil/WPIUtilJNITest.java

@@ -9,6 +9,16 @@ import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
 import org.junit.jupiter.api.Test;
 
 public class WPIUtilJNITest {
+  @Test
+  public void testEnableMockTime() {
+    assertDoesNotThrow(WPIUtilJNI::enableMockTime);
+  }
+
+  @Test
+  public void testSetMockTime() {
+    assertDoesNotThrow(() -> WPIUtilJNI.setMockTime(0L));
+  }
+
   @Test
   public void testNow() {
     assertDoesNotThrow(WPIUtilJNI::now);