Long delay when validator shows "Updating ignored errors"

[GerdP]

When you run the validator with "information level" enabled (other) it is likely to get thousands of entries. These entries are sorted using TestError.compare() which is quite costly.
On my machine it requires more than 3 seconds to sort a list of ~8850 errors like this.

[GerdP]

In 14768/josm:

fix #17302: reduce sort time in ValidatorTreePanel.buildTree()

[GerdP]

Please review: TestError still implements Comparable<TestError> although this should no longer be needed. I did not dare to remove it because we have some plugins which use the class TestError. Will have a look later.

[GerdP]

possible code simplification

[GerdP]

I found that only the plugins pt_assistent and wikipedia refer to class TestError. I was able to compile both plugins with the attached patch. Can I simply commit it or is it better to keep the legacy code?