Extract some codegen tests to black box testData

compiler/testData/codegen/box/extensionFunctions/simple.kt

@@ -0,0 +1,5 @@
+fun StringBuilder.first() = this.charAt(0)
+
+fun foo() = StringBuilder("foo").first()
+
+fun box() = if (foo() == 'f') "OK" else "Fail ${foo()}"

compiler/testData/codegen/box/extensionFunctions/whenFail.kt

@@ -0,0 +1,24 @@
+fun StringBuilder.takeFirst(): Char {
+  if (this.length() == 0) return 0.toChar()
+  val c = this.charAt(0)
+  this.deleteCharAt(0)
+  return c
+}
+
+fun foo(expr: StringBuilder): Int {
+  val c = expr.takeFirst()
+  when(c) {
+    0.toChar() -> throw Exception("zero")
+    else -> throw Exception("nonzero" + c)
+  }
+}
+
+fun box(): String {
+  try {
+    foo(StringBuilder())
+    return "Fail"
+  }
+  catch (e: Exception) {
+    return "OK"
+  }
+}