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/Fir2IrClassifierStorage.sig.kt.txt

@@ -148,7 +148,6 @@ class FirSession {
 //   Mangled name: Fir2IrComponents
 //   Public signature: /Fir2IrComponents|null[0]
 interface Fir2IrComponents {
-
   // CHECK:
   //   Mangled name: Fir2IrComponents{}classifierStorage
   //   Public signature: /Fir2IrComponents.classifierStorage|-9190759179670631614[0]