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/declarations/contextReceivers/fromKEEP/monoidSum.sig.kt.txt

@@ -33,7 +33,6 @@ interface Monoid<T : Any?> : Semigroup<T> {
 //   Mangled name: Semigroup
 //   Public signature: /Semigroup|null[0]
 interface Semigroup<T : Any?> {
-
   // CHECK JVM_IR:
   //   Mangled name: Semigroup#combine@1:0(1:0){}1:0
   //   Public signature: /Semigroup.combine|-3517256775590682507[0]