fusefs: add a test for the max_read= mount option

When set, this limits the amount of data that the kernel will request of
the server in any single read operation. The option has always been
available in our fusefs implementation, but never covered by the test
suite.

MFC after: 2 weeks
Sponsored by: ConnectWise

(cherry picked from commit 9f31c47460412ab6ccae36a70ca019b47423ccec)