#3772: wrapped with await().untilAsserted

[chickenchickenlove]

Wrapped Assertions.assertThat(disposable.isDisposed()).isTrue() with Awaitility.await().
Because disposable.dispose() is not finished yet right after latch reaches zero.

Fixes #3772

[chemicL]

Thanks for the contribution! I think it's a good approach. Please rebase to target the 3.6.x branch and let's merge it 🚀

[chickenchickenlove]

@chemicL Thanks for your comment 🙇‍♂️
I executed this command to rebase to 3.6.x.
Please take a look when you have free time!

$ git remote add upstream https://github.com/reactor/reactor-core.git
$ git checkout -b 3.6.x upstream/3.6.x
$ git checkout 240330-test-fix
$ git rebase 3.6.x # conflict occurs at gradle.properties (i resolved it)
$ git push --force

[chemicL]

Thanks! I changed the base and I think there won't be any issue, but next time consider using --onto as an argument to rebase. Check here for justifications. When the tests pass we can merge it :)

[chickenchickenlove]

I see. thanks for your kindful comments 🙇‍♂️

[chemicL]

Thank you for the contribution @chickenchickenlove 🚀