FIR JS: add K2 variants of all other JS tests

except tests that are not possible to add without some modifications in
the test infra. See todos on the commented-out test declarations

compiler/testData/codegen/box/ir/clashingFakeOverrideSignatures.kt

@@ -1,5 +1,5 @@
 // TARGET_BACKEND: JVM
-// IGNORE_BACKEND_K2: JVM_IR
+// IGNORE_BACKEND_K2: JVM_IR, JS_IR
 // FIR status: validation failed. TODO decide if we want to fix KT-42020 for FIR as well
 
 open class Base<T> {

compiler/testData/codegen/box/ir/serializationRegressions/varAsFunctionCall.kt

@@ -1,4 +1,4 @@
-// IGNORE_BACKEND_K2: JVM_IR
+// IGNORE_BACKEND_K2: JVM_IR, JS_IR
 // FIR status: ARGUMENT_TYPE_MISMATCH at ok.z(fail), actual String, expected T
 // MODULE: lib
 // FILE: l1.kt