By default, a single file runs directly, instead of in a subprocess,
and stderr is not checked. That's both more in line with the old
behavior and more suitable for DrDr's use in running an individual
test.
Also, get rid of the `drdr` submodule, which doesn't look like a good
idea anymore.
Run tests in separate processes, support tests in
parallel, flag tests with non-zero exit codes or
stderr output as failing, add timeout support, etc.
Use the `--drdr` flag as a shorthand for DrDr-like flags.
The `--drdr` flag causes `raco test` to check for a `drdr`
submodule, then a `test` submodule, then run the module
directly. (The idea is that DrDr will eventualy try the
same sequence.) A test can declare an alternate timeout
through a `config` sub-submodule (and the idea is that
"props" will go away).
