Migrate boxInline tests to new multi-file framework

compiler/testData/codegen/boxInline/special/stackHeightBug.kt

@@ -0,0 +1,19 @@
+// FILE: 1.kt
+
+package test
+
+inline fun <R> mfun(f: () -> R) {
+    f()
+    f()
+}
+
+public inline fun String.toLowerCase2() : String = ""
+
+// FILE: 2.kt
+
+import test.*
+
+fun box(): String {
+    mfun{ "".toLowerCase2() }
+    return "OK"
+}