Update tests to new `given` syntax

[nicolasstucki]

Update to new given syntax in https://github.com/scala/vscode-scala-syntax/blob/master/tests/unit/given.test.scala

[nicolasstucki]

See http://dotty.epfl.ch/docs/reference/contextual/givens.html

[nicolasstucki]

Also see scala/scala3#10538