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/castFromAny.sig.kt.txt

@@ -2,7 +2,6 @@
 //   Mangled name: KRunnable
 //   Public signature: /KRunnable|null[0]
 fun interface KRunnable {
-
   // CHECK:
   //   Mangled name: KRunnable#invoke(){}
   //   Public signature: /KRunnable.invoke|-4663091332620260873[0]