SBT test - test only files in a certain subdirectory

Say I have a directory all that contains directories with subject-specific testcases, e.g. basic, sets, issues, ...

In sbt, I can use test to run all testcases in all subdirectories of all.

I can use test-only -- -n TESTCASE to only test the TESTCASE file. What I cannot seem to figure out, though, is what arguments I must pass to what command in order to only test all the testcases in a certain subdirectory.

Any help?