test-in-container: Moving a test to a particular location *before* execution?

[DJ Delorie]

You mean something that copies the test to a specified location, and
execs that instead of the in-build one?

I suppose the easiest way to implement that is to add yet another
command to the setup script, like this:

  exec /bin/foo

which means "whatever you were about to exec, copy it to this path, and
exec that instead"

Bonus: if the destination path ends in "/", append the basename of the
specified test.