Regression in Webflux test when explicitely providing tracing headers

[beltram]

Describe the bug
Re-opening spring-projects/spring-framework#24191
Excerpt: in a junit test targeting a WebFlux controller, after migrating to spring-test 5.2.2.RELEASE, a new Span is created by TraceWebFilter regardless the ones provided in the WebTestClient by propagation headers because Reactor's context lacks original Span.

Root cause of the issue is spring-projects/spring-framework#23936.
Happens in Sleuth 2.2.0.RELEASE. Discriminant is spring-test version : works with 5.2.1.RELEASE and fails with 5.2.2.RELEASE

Sample
Reproducer is here