fstests: In TestFsPutError reliably provoke test failure
fstests: In TestFsPutError reliably provoke test failure

This change to go1.11 causes the TestFsPutError test to fail

https://go-review.googlesource.com/c/go/+/114316

This is because it now passes the half written file to the backend
whereas it didn't previously because of the buffering.

In this commit the size of the data written was increased to 5k from
50 bytes to provoke the test failure under go1.10 also.

olihey/rclone,B4dM4n/rclone,B4dM4n/rclone,ncw/rclone,B4dM4n/rclone,olihey/rclone,ncw/rclone,ncw/rclone,olihey/rclone,olihey/rclone,B4dM4n/rclone,ncw/rclone