Add multi-stage regression test

tests/run-staging/multi-staging.check

@@ -0,0 +1,5 @@
+stage1 code: ((qctx1: scala.quoted.QuoteContext) ?=> {
+  val x1: scala.Int = 2
+  scala.internal.quoted.CompileTime.exprQuote[scala.Int](1.+(scala.internal.quoted.CompileTime.exprNestedSplice[scala.Int](qctx1)(((evidence$5: qctx1.Nested) ?=> scala.quoted.Expr.apply[scala.Int](x1)(evidence$5, scala.quoted.Liftable.IntIsLiftable[scala.Int]))))).apply(using qctx1)
+})
+3

tests/run-staging/multi-staging.scala

@@ -0,0 +1,23 @@
+
+import scala.quoted._
+import scala.quoted.staging._
+
+object Test {
+  def main(args: Array[String]): Unit =
+    val s1: QuoteContext ?=> Expr[Int] = {
+      given Toolbox = Toolbox.make(getClass.getClassLoader)
+      run[QuoteContext ?=> Expr[Int]] { stage1('{2}) }
+    }
+    {
+      given Toolbox = Toolbox.make(getClass.getClassLoader)
+      println(run(s1))
+    }
+  def stage1(x: Expr[Int])(using qctx: QuoteContext): Expr[QuoteContext ?=> Expr[Int]] =
+    val code = '{ (using qctx1: QuoteContext) =>
+      val x1 = $x
+      '{ 1 + ${Expr(x1)} }
+    }
+    println("stage1 code: " + code.show)
+    code
+
+}