Add baggage test to AbstractHttpServerTest

[trask]

Add a baggage test to AbstractHttpServerTest, probably could leverage https://github.com/open-telemetry/opentelemetry-java-contrib/tree/main/baggage-processor#usage-with-sdk-auto-configuration (which is included in the Java agent already) in order to stamp the baggage on to the spans which could be used in the verification step.

A second baggage test could be added that sends two headers in order to test #12868.

[xiepuhuan]

Please assign it to me.

[steverao]

It seems that the issue was solved and close it now. If there is any problem, please reopen again.