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/superCallsComposed.sig.kt.txt

@@ -68,7 +68,6 @@ class Derived : Base, BaseI {
 //   Mangled name: BaseI
 //   Public signature: /BaseI|null[0]
 interface BaseI {
-
   // CHECK:
   //   Mangled name: BaseI#foo(){}
   //   Public signature: /BaseI.foo|-1041209573719867811[0]