Add tests cases for Unit

Author: nicolasstucki

tests/run/quote-compile-constants.check

@@ -10,3 +10,6 @@ class scala.quoted.Exprs$ValueExpr
 class scala.quoted.Exprs$ValueExpr
 class scala.quoted.Exprs$ValueExpr
 class scala.quoted.Exprs$ValueExpr
+class scala.quoted.Exprs$ValueExpr
+class scala.quoted.Exprs$ValueExpr
+class scala.quoted.Exprs$ValueExpr

tests/run/quote-compile-constants.scala

@@ -14,5 +14,8 @@ object Test {
     println(('(4.0f)).getClass)
     println(('(5.0d)).getClass)
     println(('("xyz")).getClass)
+    println(('()).getClass)
+    println(('{()}).getClass)
+    println(('{{()}}).getClass)
   }
 }