[FIR Test] Migrate AbstractFirLoadCompiledKotlin to new test infrastructure

Also introduce two different modes for those tests:
- load metadata compiled with K1
- load metadata compiled with K2

compiler/testData/loadJava/compiledKotlinWithStdlib/contracts/withReceiver.fir.txt

@@ -0,0 +1,5 @@
+public final fun R|kotlin/Any?|.isNotNull(): R|kotlin/Boolean|
+    [R|Contract description]
+     <
+        Returns(TRUE) -> this != null
+    >