Move blackBoxFile() testData to box/ directory

Delete all test methods (and empty test classes), since they'll be
auto-generated

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

@@ -0,0 +1,13 @@
+fun <T> T.mustBe(t : T) {
+    assert("$this must be $t") {this == t}
+}
+
+inline fun assert(message : String, condition : () -> Boolean) {
+    if (!condition())
+        throw AssertionError(message)
+}
+
+fun box() : String {
+    "lala" mustBe "lala"
+    return "OK"
+}