Have workcache::test clean up after itself.

[lkuper]

This just removes the stray "foo.c" that workcache::test leaves behind when, for example, make check runs.

[alexcrichton]

Can you do this in a finally block actually? That would ensure that even if a failure occurs that the file gets removed. Another possibility would be removing it in a destructor, so many possibilities!

[lkuper]

@alexcrichton OK, I just updated the patch to yet another possibility: just have the test put foo.c (and foo.o as well) in the same directory it's running in (i.e., the build directory), which was what was already happening with db.json. This solves the problem of having stray files left in one's working copy, and it's more consistent. (It turns out the foo.o was there in my working copy before, too, but I didn't notice it because it was gitignored.)