Use `async def` for tests in src/allmydata/test/mutable/test_version.py

[exarkun]

Since Twisted 22.8 we can write async def test methods (with either trial or testtools).

Let's do some of that.

The transformation is basically mechanical.

def test_foo(self):
   d = some_deferred()
   d.addCallback(lambda x: some_function(x))
   return d

becomes either

async def test_foo(self):
    x = await some_deferred()
    some_function(x)

or

async def test_foo(self):
    x = await some_deferred()
    await some_function(x)

Deferred.addCallback(f) is happy whether f returns a Deferred or not but await f() only works if f returns an awaitable - thus the choice to make above.

[exarkun]

​https://github.com/tahoe-lafs/tahoe-lafs/pull/1234

[GitHub <noreply@…>]

In 80caf04/trunk:

Merge pull request #1234 from tahoe-lafs/3947.mutable-test_version-async-def

Convert some tests for mutables to use async def

Fixes: ticket:3947