Retry attribute should not derive from PropertyAttribute

[rprouse]

The RetryAttribute derives from PropertyAttribute and passes the retry count to the base as the property to set which is never used. This sets unneeded properties on the test and will end up creating odd categories in test runners like Visual Studio.

        public RetryAttribute(int tryCount) : base(tryCount)
        {
            _tryCount = tryCount;
        }

This attribute should probably just derive from NUnitAttribute

[CharliePoole]

I think it's that way only because V2 had the property, which it actually used.

[JerryBLi]

I just submitted a pull request to fix this issue.