IR text tests: Stable blank lines between declarations in class

Rework rendering of kt-like dump and signatures dump in order to avoid
unstable blank line between declarations of the same level:
1. No blank line for the first declaration inside the member scope of
the class.
2. Always a single blank line between each two subsequent declarations
inside the member scope of the class.

compiler/testData/ir/irText/classes/enumEntries.kt

@@ -1,5 +1,3 @@
-// !LANGUAGE: +EnumEntries
-// TARGET_BACKEND: JVM_IR
 // FULL_JDK
 // WITH_STDLIB
 
@@ -7,5 +5,4 @@ enum class MyEnum {
     Ok, Nope
 }
 
-@OptIn(ExperimentalStdlibApi::class)
 fun box() = MyEnum.entries