LU-10934 tests: increase timeout for sanityn test_51b

LU-10934 tests: increase timeout for sanityn test_51b

Increase the timeout for sanityn test_51b, since this is causing
intermittent test failures when stat() is run before dd finishes.

Test-Parameters: trivial testlist=sanityn env=ONLY=51b,ONLY_REPEAT=100
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: Ieb3d50e6b534b535e8255cbbc566f053f33ebbe5
Reviewed-on: https://review.whamcloud.com/38947
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: James Nunez <jnunez@whamcloud.com>
Reviewed-by: Yingjin Qian <qian@ddn.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>