test/filestore: added check of return value of mkdir()

check return value of mkdir

Signed-off-by: Xing Lin <xinglin@cs.utah.edu>
test/filestore: add check of return values in StoreTest::SetUp

add return value check for function calls, including mkdir, mkfs
and mount

Signed-off-by: Xing Lin <xinglin@cs.utah.edu>