Move simple ccall tests out of cheritest.c into their own cheritest_ccall.c.
Move libcheri-related tests to their own file in cheritest.
Fix style, update manpage for sandbox_class_getlength().
Use MK_MAN=no rather than NO_MAN=yes for cheritest-related Makefiles to
silence a new build warning.
Move cheri_fd_new() in cheritest to cheritest_libcheri.c close to other
libcheri-related initalisation.