AbstractEventProcessorTest: Make inner class static

When all error prone warnings are enabled the ClassCanBeStatic
bug pattern is reported:

  plugins/webhooks/src/test/java/com/googlesource/gerrit/plugins/webhooks/processors/AbstractEventProcessorTest.java:87:
  error: [ClassCanBeStatic] Inner class is non-static but does not reference enclosing class
  private class TestEventProcessor extends AbstractEventProcessor {
          ^
    (see https://errorprone.info/bugpattern/ClassCanBeStatic)

Change-Id: I2059a17858581fbb6215f1cf194e05216ad4a130

src/test/java/com/googlesource/gerrit/plugins/webhooks/processors/AbstractEventProcessorTest.java

diff --git a/src/test/java/com/googlesource/gerrit/plugins/webhooks/processors/AbstractEventProcessorTest.java b/src/test/java/com/googlesource/gerrit/plugins/webhooks/processors/AbstractEventProcessorTest.java
index 807f7b0..e4056b4 100644
--- a/src/test/java/com/googlesource/gerrit/plugins/webhooks/processors/AbstractEventProcessorTest.java
+++ b/src/test/java/com/googlesource/gerrit/plugins/webhooks/processors/AbstractEventProcessorTest.java
@@ -84,7 +84,7 @@
     assertThat(actual).isFalse();
   }
 
-  private class TestEventProcessor extends AbstractEventProcessor {
+  private static class TestEventProcessor extends AbstractEventProcessor {
     @Override
     public Optional<EventProcessor.Request> doProcess(ProjectEvent event, RemoteConfig config) {
       // do nothing