fuse/runtests: added linear write tests for various block sizes

fuse/runtests: added linear write tests for various block sizes

unit tests to test writing contiguous blocks linearly through the file,
for a variety of block sizes;  'tiny_file' is an entire file fitting within
a single io block / write operation.  'linear_{small,large}_writes' test
a 1Mb file written with each write operation containing significantly less
or more, respecitvely, data than fuse will pass into the implementation as
a single operation (which on the mac at least is 64Kib)