FIR IDE: add tests for completion in sealed when

idea/idea-fir/testData/completion/basic/common/when/nonSealedClassInWhen.kt

@@ -0,0 +1,21 @@
+abstract class SEALED
+class AAAA: SEALED()
+object BBBB: SEALED()
+class CCCC: SEALED()
+
+class SomeClass
+object SomeObject
+
+fun foo(e: SEALED) {
+    when (e) {
+        <caret>
+    }
+}
+
+// EXIST: is AAAA
+// EXIST: BBBB
+// EXIST: is CCCC
+// EXIST: is SomeClass
+// EXIST: SomeObject
+// EXIST: { lookupString: "else -> "}
+// FIR_COMPARISON