Update cheritest to use 'sandbox_class' and 'sandbox_object' instead of
'sandbox', and reuse sandbox instances between test invocations.  Register
cheritest-helper methods so that 'procstat -R' can properly list them.

(Ideally method names would be extracted from ELF metadata, but we're not
there yet.)

Use common invocation code for 'spin' and 'syscall' methods, which aren't
more special than most of the others.