Fix RedundantInterfaces Serializable if check

[vanniktech]

The interface name comes with dots and not slashes.

[amaembo]

Please add to findBugsTestCases a test case which failes without your fix, but succeeds with it. You may name your test as /findbugsTestCases/src/java/ghIssues/Issue0049.java. Use edu.umd.cs.findbugs.annotations.NoWarning and/or edu.umd.cs.findbugs.annotations.ExpectWarning annotations.

[vanniktech]

@amaembo sorry it took me that long. I added a test file. Though I don't know how to run the tests or if it's even in the right format.

[amaembo]

I don't understand this test: why public static class SuperClass implements TestInterface {} should not produce the warning, but public static class SubClassWithDuplicate implements TestInterface {} should produce? What's the difference between these two?

[vanniktech]

@amaembo you're right. I forgot to subclass. Now the tests should make sense. I also squashed the commits together into one.

[amaembo]

Merged, thanks.