Replace serial scalatest ExecutionContext with global

[custommonkey]

Fix for #145

[djspiewak]

Do we also need to remove the createIORuntime helper? I only vaguely remember some of this stuff. I do agree with removing the serial executor though. If people want that semantic, TestControl exists and does the job better.

[custommonkey]

createIORuntime is used in minitest so it can't go completely and updating AsyncIOSpec to not extend RuntimePlatform is a binary breaking change so that seems a little harsh.

ATM I'm trying to figure out why it works with the jvm, but no js. It seems scalatest has specific behaviour for js https://github.com/scalatest/scalatest/blob/7aeda262e249a66d2319b6b2f5daa1b7d0cf9a4f/jvm/core/src/main/scala/org/scalatest/concurrent/SerialExecutionContext.scala#L118 so I might need to use a variation to provide the correct runtime.

[djspiewak]

Ooooooh, ScalaTest just doesn't let you use alternative executors on JavaScript. That's actually really annoying.