Can we create playmode tests outside of the main build assembly?

I just tried to create my first playmode test. It is the public IEnumerator with [UnityTest] method which i put in my Editor folder(and thus assembly) where all my editor tests reside. However, unity test runner can't see it.

It does find the tests which are in my Assets folder, in the Assembly-CSharp. This also have added the NUnit framework (and, interestingly, my NSubstiture and FluentAssertions assemblies, which are configured to only be included in Editor platform):

So i wonder: can i separate the tests into the Editor assembly altogether? Does the additional assemblies and test files slow the main build(when unity compiles files when i focus the window or hit the play button)?