Test: add java source files as JavaSourceRoot

so that those can be properly populated in SingleJavaRootsIndex

compiler/testData/diagnostics/tests/j+k/differentFilename.kt

@@ -1,3 +1,4 @@
+// FIR_IDENTICAL
 // SKIP_JAVAC
 // FILE: A.java
 public class A {
@@ -20,7 +21,7 @@ class F {
 // FILE: main.kt
 fun main(x: A) {
     x.b().bar()
-    x.<!MISSING_DEPENDENCY_CLASS!>f<!>().<!UNRESOLVED_REFERENCE!>foobaz<!>()
+    x.f().foobaz()
 
-    <!UNRESOLVED_REFERENCE!>D<!>().<!DEBUG_INFO_MISSING_UNRESOLVED!>baz<!>()
+    D().baz()
 }