Add special span tag for async test cases

[Kyle-Verhoog]

Currently async test cases can only be implemented by attaching a separate http header but this is non-trivial to do in some languages (java, .net)

An alternative to the http header would be to have a special test token span tag that the test agent looks for to associate traces to test cases. (eg. DD_TAGS=_testagent_token: test.class.test_method)

[Kyle-Verhoog]

The question would be whether or not the tag should be included in the snapshot output when snapshotting.