Disable frivolous PMD warnings and enable PMD in ntcore (#3419)

Some valid warnings like throwing NullPointerException or using a for
loop instead of System.arraycopy() were fixed.

Abstract classes marked with PMD.AbstractClassWithoutAbstractMethod were
made concrete because they already had protected constructors.

Fixes #1697.

wpilibNewCommands/src/test/java/edu/wpi/first/wpilibj2/command/CommandTestBase.java

@@ -13,8 +13,9 @@ import java.util.Set;
 import org.junit.jupiter.api.BeforeEach;
 
 /** Basic setup for all {@link Command tests}." */
-@SuppressWarnings("PMD.AbstractClassWithoutAbstractMethod")
 public abstract class CommandTestBase {
+  protected CommandTestBase() {}
+
   @BeforeEach
   void commandSetup() {
     CommandScheduler.getInstance().cancelAll();

wpilibNewCommands/src/test/java/edu/wpi/first/wpilibj2/command/MecanumControllerCommandTest.java

@@ -87,7 +87,6 @@ class MecanumControllerCommandTest {
 
   @Test
   @ResourceLock("timing")
-  @SuppressWarnings("PMD.AvoidInstantiatingObjectsInLoops")
   void testReachesReference() {
     final var subsystem = new Subsystem() {};
 

wpilibNewCommands/src/test/java/edu/wpi/first/wpilibj2/command/SwerveControllerCommandTest.java

@@ -82,7 +82,6 @@ class SwerveControllerCommandTest {
 
   @Test
   @ResourceLock("timing")
-  @SuppressWarnings("PMD.AvoidInstantiatingObjectsInLoops")
   void testReachesReference() {
     final var subsystem = new Subsystem() {};