Standardize on `//> using` in tests.

[odersky]

Tests currently use the // scala: ... syntax to add an option to the compilation. Example:

// scalac: -Wunused:privates

They should be using //> using instead. We should reinforce conventions rather than creating alternative ones.

I think this would be a good issue for the spree.

[dwijnand]

To be fair, // scalac predates the using one...

But, pedantics aside, I'm strongly in favour!

[odersky]

Ah yes, I mis-spoke. It's just that I came across // scalac: only today.

[lrytz]

(Scala 2: scala/scala#10405)

[som-snytt]

There are test-only pragmas that it would be nice to adopt or make standard, such as to express "this test should be run only under certain JVM versions" (as opposed to requesting a specific JVM version).

Maybe just a tweak such as

//> using jvm 17+

or maybe support for test tags or groups. I haven't used scala-cli test command yet, but presumably any test should be runnable that way.

The colon syntax in dotty supports:

/** Famous tool names in the ecosystem. Used for tool args in test files. */
enum ToolName:
  case Scala, Scalac, Java, Javac, ScalaJS, Test

[mbovel]

Is this issue solved by #18413?