fuse/runtests: added a 'todo' flag, surpressing failure for implementations not expec...

fuse/runtests: added a 'todo' flag, surpressing failure for implementations not expected to pass

since the current tests assume that the implementation responds to changes made
to tahoe after mount, and impl_b prefetches and cached directory data, impl_b
fails the current 'read' test suite.

rather than reflect that problem in the overall failure of the runtests exit
code, this adds a 'todo' flag to the implementations table, and sets the todo
flag for impl_b.  Thus errors will therein be reported in output, but not cause
a failing exit code.