Rename *.jet -> *.kt in codegen tests

compiler/testData/codegen/controlStructures/ifBlock.kt

@@ -0,0 +1,10 @@
+import java.util.*
+
+fun concat(l: List<String>): String? {
+    val sb = StringBuilder()
+    for(s in l) {
+        val x = if(l.size() > 1) { "T" } else { "F" };
+        sb.append(x)
+    }
+    return sb.toString()
+}