Investigate proc macro tests failing locally

[integraledelebesgue]

Tests introduced in #370 work well in CI but fail in the local environment with an error saying that macros used in tests are unavailable. Probably there's an issue with launching multiple instances of Proc Macro Server.

At the moment, those tests are compiled conditionally with test_proc_macros feature but in the future, we would like to have them turned on unconditionally.