Fix single scalar hydrator memory leak on exception

[olsavmic]

Resolves #8482, Closes #7315

I have encountered an issue with a memory leak in SingleScalarHydrator.

AbstractHydrator::hydrateAll() method does not take into account the possibility of an exception being thrown so when SingleScalarHydrator::hydrateAllData throws NoResultException/NonUniqueResultException, the event listener for onClear event is not removed and the hydrator is stuck in the memory.

It's been solved here once but not merged due to missing test cases.

Original issue: Fix memory leak in AbstractHydrator

I'll try to provide a fix and appropriate test case for this issue so it can be finally merged.

[greg0ire]

self::at is deprecated in recent versions of PHPUnit, please avoid it

[olsavmic]

Fixed and rebased, although I do not like the replacement (I did not find any other recommended way). Say if it seems ok to you please. I might replace the the rest of self::at occurrences in this Test class if requested (although it's out of scope of this MR)

[greg0ire]

I might replace the the rest of self::at occurrences in this Test class if requested (although it's out of scope of this MR)

It would certainly be appreciated, but it's not at all required.

[greg0ire]

Please kindly squash your commits together. If you don't, we'll try to remember to do it for you but it's best if you save us this trouble.

How to do that?

1. git rebase -i origin/2.8.x, assuming origin is a git remote that points to this repository, and not your fork. If you're not sure what your remotes are, run git remote -vvv, there should be your fork and the holy/reference/base/origin/whatever-you-call-it repository.
2. A window will show up with many lines, replace pick with fixup on every line but the first one
3. Close your editor, git should do its magic, and you should end up with one commit
4. Use git push --force to overwrite what you already push. Don't forget the --force option otherwise git will try to merge both things together.

[greg0ire]

Thanks @olsavmic !