Review the slow/quick distinction

[samoht]

It could be useful to have something more powerful, with custom tags and a simple query language. Also, on simple projects nobody care about that feature so we should not make it mandatory.

[emillon]

A way to do that is to group the slow tests in a different executable, so that one can use dune runtest tests/fast/ to only run fast ones (or do not add the slow ones to @runtest).