IR text tests: Unmute tests that have main function

Don't use `main` function in these tests, because `main` function
has specific mangling rules in JVM. Use other function name.

^KT-57755

compiler/testData/ir/irText/firProblems/AssignmentOperator.kt

@@ -1,5 +1,3 @@
-// MUTE_SIGNATURE_COMPARISON_K2: JVM_IR
-// ^ KT-57755: Fix computing a mangled name for the `main` function
 package foo
 
 class A {
@@ -16,7 +14,7 @@ fun <T> id(t: T): T {
     return t
 }
 
-fun main() {
+fun runMe() {
     val a = A()
 
     id(a).prop = 10