This substitution effectively duplicated enclosing RUN command for each file in the given path. Accepts wildcards.
In order to keep implementation simple, only a single such substitution per command is supported. It is possible to support multiple such substitution via cartesian product, if deemed necessary.
Use case for this is upcoming refactoring of C++ DR tests structure to avoid tests interfering with each other. Currently, tests are grouped by 100 per file, and have a RUN command per C++ Standard revision (for a total of 7 with the recent addition of c++2c mode). The plan is to rely heavily on split-file to invoke the compiler individually for each test, but avoid duplicating RUN commands for each of them. More details in Discourse thread.