Ignore compiletime.testing for now

[Cherry-picked 4d28ffd]

Author: som-snytt

compiler/src/dotty/tools/dotc/transform/CheckUnused.scala

@@ -473,6 +473,7 @@ object CheckUnused:
         if inliners == 0
           && languageImport(imp.expr).isEmpty
           && !imp.isGeneratedByEnum
+          && !imp.isCompiletimeTesting
           && !ctx.owner.name.isReplWrapperName
         then
           if imp.isCompiletimeTesting then