Add some methods to build client async in TestUtils

[netstart]

methods to build client async in TestUtils
Some async tests

[whummer]

Thanks for this PR @netstart ! Can we first merge PR #839 , and then rebase this branch onto the changes made in #839 ? Would be great to merge in the entire set of changes, without running into conflicts. Thanks

[coveralls]

Coverage increased (+0.01%) to 69.363% when pulling 603ae2d on netstart:master into 56b1cfe on localstack:master.

[netstart]

It's OK to me. I thought that 839 PR was forgotten,

[whummer]

@netstart Sorry, maybe my last message was not clear. Wasn't your PR adding some additional capabilities, in addition to #839 ? Should we re-open this PR, resolve the conflicts, and get it merged? Thanks

[netstart]

I Understood, thank you for your atemption!
Don't worry, I will send other PR ;)