Generify Naggum.Assembler tests

[ForNeVeR]

Currently there are handwritten tests such as the folliwing:

[<Fact>]
let ``Hello world program should be executed`` () =
    let source = "(.assembly Hello
  (.method Main () System.Void (.entrypoint)
    (ldstr \"Hello, world!\")
    (call (mscorlib System.Console WriteLine (System.String) System.Void))
    (ret)))
"
    let output = execute source
    Assert.Equal ("Hello, world!\n", output)

We should make the test process more generic (as Naggum.Compiler testing already done): it should read all *.nga files from the test directory, compile them, then execute and verify its outputs and/or return codes against some expected values.

Maybe we could even set up the common infrastructure for both Naggum.Assembler and Naggum.Compiler testing (and detect compiler type by file extension).