Add test for async utest TestSuites

Sometimes when utest runs async tests (i.e. tests that return a future)
the test suite will fail even when none of the individual tests do. This
is due to a data race in sbt. Most, but not all, of the time, this test
will induce that race.

sbt/src/sbt-test/tests/fork-async/build.sbt

@@ -0,0 +1,8 @@
+testFrameworks += new TestFramework("utest.runner.Framework")
+
+lazy val root = (project in file(".")).
+  settings(
+    scalaVersion := "2.12.4",
+    libraryDependencies += "com.lihaoyi" %% "utest" % "0.6.4" % Test,
+    fork in Test := true
+  )

sbt/src/sbt-test/tests/fork-async/src/test/scala/ForkAsyncTest.scala

@@ -0,0 +1,21 @@
+import utest._
+
+import utest._
+import utest.framework._
+
+import scala.concurrent.{ ExecutionContext, Promise }
+import scala.util.Success
+
+object ForkAsyncTest extends TestSuite {
+  val g = ExecutionContext.global
+  val n = 10
+  val (testNames, promises) = (1 to n).map(i => Tree(s"$i") -> Promise[Unit]).unzip
+  val testTrees = promises.zipWithIndex.map { case (p, i) =>
+    new TestCallTree(Left {
+      if (i == (n - 1)) promises.foreach(p => g.execute(() => p.tryComplete(Success(()))))
+      p.future
+    })
+  }
+  val tests =
+    Tests(nameTree = Tree("async", testNames: _*), callTree = new TestCallTree(Right(testTrees)))
+}

sbt/src/sbt-test/tests/fork-async/test

@@ -0,0 +1 @@
+> test