[java-highlighting] Refactor FunctionalInterfaceTest to avoid internals dependency; move completion stuff out of LambdaHighlightingUtil

Part of IDEA-365344 Create a new Java error highlighter with minimal dependencies (PSI only)

GitOrigin-RevId: 4bd5447dd5fc2342f5e9b23e693f7683594470f5

java/java-tests/testData/codeInsight/daemonCodeAnalyzer/lambda/functionalInterface/Clone.java

@@ -1,3 +1,4 @@
+<error descr="Multiple non-overriding abstract methods found in Foo">@FunctionalInterface</error>
 interface Foo {
   int m();
   Object clone();