airspec: Allow reusing test specs

To reuse test functions, we can use the return value of test method like this:

Idea A

class Fixture[A](data:Seq[A]) extends AirSpec {
   def test1: Unit = { data shouldNotBe empty  }
   def test2: Unit = { data.length shouldBe data.size }
}

class MyTest extends AirSpec {
   def testIntSeq: AirSpec = new Fixture(Seq(1, 2, 3)) // Run the returned spec
   def testStringSeq: AirSpec = new Fixture(Seq("a", "b")) // Run the returned spec
}

If Seq[AirSpec] is returned, it should run this sequence of test specs.

idea B Another approach is passing a test context through DI, then allow running new AirSpec:

def test(context:Context): Unit = {
  context.run(new Fixture(Seq(1, 2, 3)) 
  context.run(new Fixture(Seq("a", "b"))
}

• It seems idea B is more straightforward
• Technically speaking, B is better since we can get a Surface of Fixture using def run[A <: AirSpec](newSpec:A) to get a list of test methods.

[takezoe]

Thanks for your explanation. Now I get fully understanding! Finally, I prefer B for its flexibility 😃

[xerial]

Done in #602