[test] Fix dr324.c again for non-writeable source directories

In general, the source tree is not assumed to be writeable, so modifying `%s` does not work for all CI systems. Instead of touching `%s`, copy it to a writeable dir using `%t`, and touch it there.
Actually, `dr0xx.c` isn't really needed at all, so just create a new `dep.c` file in the build tree.

This was recently added in cb088e8c, fixed in 1481fcf7, and fixed again in d16c5901.