JVM IR: Avoid direct invokes in callable reference tests

Due to the direct invoke optimization, most callable reference tests
were not generating callable references/lambdas.

compiler/testData/codegen/box/multifileClasses/optimized/callableRefToFun.kt

@@ -6,7 +6,7 @@
 
 import a.*
 
-fun box(): String = (::ok)()
+fun box(): String = (::ok).let { it() }
 
 // FILE: part1.kt
 @file:[JvmName("MultifileClass") JvmMultifileClass]