Move self-test functionality into its own file.

src/core/include.am

src/core/mainloop/mainloop.c

src/core/or/circuitbuild.c

src/core/or/circuituse.c

src/feature/control/control.c

src/feature/dirclient/dirclient.c

src/feature/relay/router.c

src/feature/relay/router.h

src/feature/relay/selftest.c

src/feature/relay/selftest.h

src/feature/stats/rephist.c