[Analysis API] tests: use tagged caret to find declaration to analyze in the air against

We cannot use a KtFile as analysis context.

^KT-55527

analysis/analysis-api/testData/components/referenceShortener/referenceShortenerWholeFile/fileWithMultipleDeclarations.kt

@@ -1,4 +1,4 @@
-class A {
+class A<caret_onAirContext> {
     abstract class B {
         abstract val b: A.B
     }