* fuse/runtests: added --catch-up-pause option

* fuse/runtests: added --catch-up-pause option

On linux, write tests are failing because data written to fuse isn't showing
up in tahoe by the time it's checked.  it's not clear where this is originating,
since the fuse implementation [should be] waiting for completion of tahoe
operations before returning from its calls.  This adds an option to control the
duration of a pause between the fuse write and the check of tahoe, which is by
default set to 2s on linux, which - somewhat inexplicably - seems to 'fix' the
problem, in as far as it allows tests to complete.