Background & Motivation
The Initial DNS Seedlist Discovery specification tests that contain credentials are skipped for single-threaded tests. They test connecting to these URIs:

mongodb+srv://auser:apass@test1.test.build.10gen.cc/?replicaSet=repl0
mongodb+srv://b*b%40f3tt%3D:%244to%40L8%3DMC@test3.test.build.10gen.cc/mydb%3F?replicaSet=repl0

Currently, the single-threaded tests that expect the DNS resolution to succeed require authentication to succeed. This is because single-threaded monitoring connections are authenticated (since they are also used for application-use). The multi-threaded test cases are OK, because the background monitoring connections do not undergo authentication.

But, by skipping initial seedlist DNS tests containing credentials on single-threaded we do lose a little valuable test coverage. Single-threaded DNS resolution and SRV polling behaves slightly differently from multi-threaded.

Scope
TBD, but one possible solution is to create/drop the user for the duration of the test.