use new slash syntax in test

[xuwei-k]

prepare sbt 2.x

• https://github.com/sbt/sbt/pull/7703
• https://www.scala-sbt.org/1.x/docs/Migrating-from-sbt-013x.html#Migrating+to+slash+syntax

[mzuehlke]

Does this imply the minimal sbt versions needs to be raise to 1.5.0 ? https://github.com/scalameta/sbt-scalafmt/blob/main/build.sbt#L55-L57