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/firProblems/Modality.sig.kt.txt

@@ -66,7 +66,6 @@ abstract class ResolutionPart {
 //   Mangled name: Substitutable
 //   Public signature: /Substitutable|null[0]
 interface Substitutable<out T : DeclarationDescriptorNonRoot> {
-
 }
 
 // CHECK: