KT-61890 [AA] Add test for ContextCollector at loop variable position

This turns out to be important for conflict resolution in
postfix completion in K2 IDE (see `KotlinDeclarationNameValidator`)

analysis/low-level-api-fir/testData/contextCollector/forLoopVariable.kt

@@ -0,0 +1,5 @@
+package test
+
+fun usage(list: List<Int>) {
+    for (<expr>loopVar</expr> in list) {}
+}