TestId allows you to filter on the ID of the Test associated with a Run or assignment.
This field and the Test field can be both used for searching by TestId - but key differences are that:
Matching by single Id:
TestId = 07649a3b-f086-4d0d-94b8-a14d00850fe9
Matching by multiple Id's:
TestId IN [07649a3b-f086-4d0d-94b8-a14d00850fe9, 91f8769a-1f02-4201-9c85-a14d008e9a43]