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/plusMatrix.sig.kt.txt

@@ -14,7 +14,6 @@ class Matrix {
 //   Mangled name: NumberOperations
 //   Public signature: /NumberOperations|null[0]
 interface NumberOperations {
-
   // CHECK JVM_IR:
   //   Mangled name: NumberOperations#plus@kotlin.Number(kotlin.Number){}kotlin.Number
   //   Public signature: /NumberOperations.plus|-6518987633126930952[0]