memtester: fflush/fsync for stderr after reporting a problem

This might be a bit of a cargo cult, but when probing for optimal
dram_tpr3 values, some of the reports from memotester end up being
zero-sized. This patch might or might not help, but let's have
it anyway.

Signed-off-by: Siarhei Siamashka <siarhei.siamashka@gmail.com>