Add FIR enhancement tests, fix some exceptions / problems around them

Test data and tests themselves are based on
compiler/testData/loadJava/compiledJava

compiler/testData/loadJava/compiledJava/vararg/VarargInt.fir.txt

@@ -0,0 +1,4 @@
+public open class VarargInt : R|java/lang/Object| {
+    public open operator function vararg(vararg p: R|kotlin/IntArray|): R|kotlin/Unit|
+
+}

compiler/testData/loadJava/compiledJava/vararg/VarargString.fir.txt

@@ -0,0 +1,4 @@
+public open class VarargString : R|java/lang/Object| {
+    public open operator function vararg(vararg strings: R|kotlin/Array<ft<java/lang/String, java/lang/String?>>|): R|kotlin/Unit|
+
+}