Renamed *.jet to *.kt in test data of JetPsiCheckerTest.

idea/testData/checker/regression/Jet69.kt

@@ -0,0 +1,10 @@
+class Command() {}
+
+fun parse(<warning>cmd</warning>: String): Command? { return null  }
+
+fun Any.equals(other : Any?) : Boolean = this === other
+
+fun main(args: Array<String>) {
+    val command = parse("")
+    if (command == null) 1 // error on this line, but must be OK
+}