Check validity of the errors from latest zipkin

Check the validity of the enforcer rule when it's applied to [zipkin](https://github.com/openzipkin/zipkin).

The maven-shade-plugin may be missing JsonTreeReader.

# zipkin

```
[ERROR] Linkage Checker rule found 1 error. Linkage error report:
Class zipkin2.internal.gson.internal.bind.JsonTreeReader is not found;
referenced by 1 class file
zipkin2.internal.gson.stream.JsonReader (zipkin-2.16.2-SNAPSHOT.jar)
```

Does this produce a new linkage error? (But the package name indicate `zipkin2.internal.gson.stream.JsonReader` is only for internal use. Identify which public class touches the class.

```
static {
JsonReaderInternalAccess.INSTANCE = new JsonReaderInternalAccess() {
@Override public void promoteNameToValue(JsonReader reader) throws IOException {
if (reader instanceof JsonTreeReader) {
((JsonTreeReader)reader).promoteNameToValue();
return;
}
int p = reader.peeked;
if (p == PEEKED_NONE) {
p = reader.doPeek();
}
if (p == PEEKED_DOUBLE_QUOTED_NAME) {
reader.peeked = PEEKED_DOUBLE_QUOTED;
} else if (p == PEEKED_SINGLE_QUOTED_NAME) {
reader.peeked = PEEKED_SINGLE_QUOTED;
} else if (p == PEEKED_UNQUOTED_NAME) {
reader.peeked = PEEKED_UNQUOTED;
} else {
throw new IllegalStateException(
"Expected a name but was " + reader.peek() + reader.locationString());
}
}
};
}
```

# zipkin-tests

```
[INFO] --- maven-enforcer-plugin:3.0.0-M2:enforce (enforce) @ zipkin-tests ---
[WARNING] Linkage Checker rule found 6 errors. Linkage error report:
Class zipkin2.internal.gson.internal.bind.JsonTreeReader is not found;
referenced by 1 class file
zipkin2.internal.gson.stream.JsonReader (zipkin-2.16.2-SNAPSHOT.jar)
Class kotlin.jvm.functions.Function0 is not found;
referenced by 1 class file
org.junit.jupiter.api.AssertionsKt (junit-jupiter-api-5.5.1.jar)
Class kotlin.jvm.internal.Intrinsics is not found;
referenced by 2 class files
org.junit.jupiter.api.AssertionsKt (junit-jupiter-api-5.5.1.jar)
org.junit.jupiter.params.aggregator.ArgumentsAccessorKt (junit-jupiter-params-5.5.1.jar)
Class kotlin.jvm.internal.Lambda is not found;
referenced by 1 class file
org.junit.jupiter.api.AssertionsKt (junit-jupiter-api-5.5.1.jar)
Class kotlin.collections.CollectionsKt is not found;
referenced by 1 class file
org.junit.jupiter.api.AssertionsKt (junit-jupiter-api-5.5.1.jar)
Class kotlin.collections.ArraysKt is not found;
referenced by 1 class file
org.junit.jupiter.api.AssertionsKt (junit-jupiter-api-5.5.1.jar)
```

They are Kotlin-related classes. JUnit Jupiter is the combination of the new programming model and extension model for writing tests and extensions in JUnit 5.