[build] Enable spotbugs (#3601)

Benign spotbugs warnings were suppressed, and all others were fixed. Bug
descriptions are documented here:
https://spotbugs.readthedocs.io/en/stable/bugDescriptions.html

Co-authored-by: Austin Shalit <austinshalit@gmail.com>

wpilibOldCommands/src/test/java/edu/wpi/first/wpilibj/command/AbstractCommandTest.java

@@ -18,7 +18,7 @@ public abstract class AbstractCommandTest {
     Scheduler.getInstance().enable();
   }
 
-  public class ASubsystem extends Subsystem {
+  public static class ASubsystem extends Subsystem {
     Command m_command;
 
     @Override

wpilibOldCommands/src/test/java/edu/wpi/first/wpilibj/command/ConditionalCommandTest.java

@@ -17,7 +17,6 @@ class ConditionalCommandTest extends AbstractCommandTest {
   MockCommand m_onTrue;
   MockCommand m_onFalse;
   MockSubsystem m_subsys;
-  Boolean m_condition;
 
   @BeforeEach
   void initCommands() {