Add test methods and data for uncommon cases

Uncommon means mostly that aren't present in raw fir builder data

compiler/visualizer/testData/uncommonCases/testFiles/variance.kt

@@ -0,0 +1,16 @@
+interface Source<out T> {
+    fun nextT(): T
+}
+
+fun demo(strs: Source<String>) {
+    val objects: Source<Any> = strs
+}
+
+interface Comparable<in T> {
+    operator fun compareTo(other: T): Int
+}
+
+fun demo(x: Comparable<Number>) {
+    x.compareTo(1.0)
+    val y: Comparable<Double> = x
+}