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/expressions/funInterface/arrayAsVarargAfterSamArgument_fi.sig.kt.txt

@@ -2,7 +2,6 @@
 //   Mangled name: IRunnable
 //   Public signature: /IRunnable|null[0]
 fun interface IRunnable {
-
   // CHECK:
   //   Mangled name: IRunnable#run(){}
   //   Public signature: /IRunnable.run|-991178076551934874[0]