More tests for nested form fieldsets

This PR is related to #5093, #6098 and may be with #5640 and others.
I have made more complex test for extracting and populating values to nested form fieldsets.

Strange situation is that I have commented the changes those were made in this PR https://github.com/zendframework/zf2/pull/5093 and all form test passed without any problem.
Unfortunately,  I do not remember why I made that PR #5093 then, may be I was too newbie in ZF2 or changes made after, had fixed the issues I had been faced then.

Nevertheless, if all tests are passed without changes made in PR #5093, may be we could remove that code