Do not output empty non-root packages in tests with txt

Mostly to avoid listing every internal package in
kotlin-stdlib/kotlin-reflect, as in
https://github.com/JetBrains/kotlin/blob/acd5b62148d797ffd24c4811d1d663f0c1ae7885/compiler/testData/diagnostics/testsWithUnsignedTypes/conversions/conversionOfSignedToUnsigned.txt#L144

compiler/tests/org/jetbrains/kotlin/types/AbstractTypeBindingTest.kt

@@ -118,7 +118,5 @@ abstract class AbstractTypeBindingTest : KotlinTestWithEnvironment() {
             }
             popIndent()
         }
-
-        override fun toString(): String = out.toString()
     }
 }