Increase x86_64_multithread_test to medium+
Matches x86_64_build because it is still compiling a bit.
@@ -115,6 +115,7 @@ jobs:
docker:
# `kronicdeth` is temporary until we get a DockerHub organization
- image: kronicdeth/lumen-development
+ resource_class: medium+
environment:
CARGO_HOME: cargo
steps:
