Silently ignores tests, when inline_tests is not specified (ppx_custom_printf)

I am not sure if it is really possible to fix, or it is one of the design constrains. Please fill free to close with Will-Not-Fix.

While exploring ppx_custom_printf rewriter, I found that dune build @runtest command does almost nothing for this package (checked with strace). However, tests are written. Quite soon, it revealed that test/dune file missed mandatory inline_tests stanza. See https://github.com/janestreet/ppx_custom_printf/issues/5

The problem is that tests were silently ignored. So, the developer would think mistakenly that they have passed (after fix they passed indeed).

I don't think that there is a good solution for this problem. But, as a semi-solution dune can warn, if absolutely no tests were executed for runtest target. This definitely will not hurt.

Or, may be, dune can dump the list of executed tests? Painted in emerald green, of course.

Start by reproducing dune build @runtest for the ppx_custom_printf case and inspect the mentioned test/dune file, including its missing inline_tests stanza. Review the linked issue for context, then determine whether Dune should warn when runtest executes no tests or report the executed test list; done requires a defined behavior and coverage for the chosen approach.