Restore testing in the CI and fix scala3-staging build

[WojciechMazur]

Fixes #24799

scala3-staging was not tested, at least not since scala3-repl seperation, it actually is currently broken when using from REPL:

WIP - blocked by classpath issues
A REPL-staging test currently fails, I believe it's the same underlying issue as with REPL classpath separation and might require a larger refactor

[info] Test scala.quoted.staging.repl.StagingScriptedReplTests.replStagingTests started
  scala> import scala.quoted._
  scala> import quoted.staging.{Compiler => StagingCompiler, _}
  scala> implicit def compiler: StagingCompiler = StagingCompiler.make(getClass.getClassLoader)
  def compiler: scala.quoted.staging.Compiler
  scala> def v(using Quotes) = '{ (if true then Some(1) else None).map(v => v+1) }
  def v(using x$1: scala.quoted.Quotes): scala.quoted.Expr[Option[Int]]
  scala> scala.quoted.staging.withQuotes(v.show)
  val res0: String = "(if (true) scala.Some.apply[scala.Int](1) else scala.None).map[scala.Int](((v: scala.Int) => v.+(1)))"
  scala> scala.quoted.staging.run(v)
+ java.lang.IncompatibleClassChangeError: Class Generated$Code$From$Quoted$$Lambda/0x0000000800a50208 does not implement the requested interface scala.Function1
+   at scala.Option.map(Option.scala:244)
+   at Generated$Code$From$Quoted.apply(<quoted.Expr>:1)
+   at jdk.internal.reflect.DirectMethodHandleAccessor.invoke(DirectMethodHandleAccessor.java:103)
+   at java.lang.reflect.Method.invoke(Method.java:580)
+   at scala.quoted.staging.QuoteDriver.run(QuoteDriver.scala:70)
+   at scala.quoted.staging.Compiler$$anon$1.run(Compiler.scala:50)
+   at scala.quoted.staging.package$.run(staging.scala:19)
+   ... 75 elided
- val res1: Option[Int] = Some(2)