missing config values in string expressions are not properly handled

[hyperthunk]

If you reference a config location ${foo.bar.baz} in a configuration element and it is not found, the atom not_found is translated into place, which is almost certainly not what the test author wants when setting e.g., environment variables and flags and so on.

Perhaps should use the ?REQUIRE macro when unpacking string expressions? Either way, the expression should not be evaluated and we should explicitly fail (with throw/1 or error/1) when this happens, so that the user gets a decent indication of what went wrong.