LU-16752 test: improve sanity 413a/b reliability

LU-16752 test: improve sanity 413a/b reliability

Set qos_maxage to 1 early in test_qos_mkdir() to ensure statfs are
updated in round-robin mkdir test, so that the subsequent QoS mkdir
behave as expected.

Test-Parameters: trivial
Test-Parameters: mdscount=2 mdtcount=4 testlist=sanity
Test-Parameters: mdscount=2 mdtcount=4 testlist=sanity
Test-Parameters: mdscount=2 mdtcount=4 testlist=sanity
Test-Parameters: mdscount=2 mdtcount=4 testlist=sanity
Test-Parameters: mdscount=2 mdtcount=4 testlist=sanity
Test-Parameters: mdscount=2 mdtcount=4 testlist=sanity
Test-Parameters: mdscount=2 mdtcount=4 testlist=sanity
Test-Parameters: mdscount=2 mdtcount=4 testlist=sanity
Fixes: 233344d451 ("LU-13417 test: generate uneven MDTs early for sanity 413")
Fixes: c1d0a355a6 ("LU-12624 lod: alloc dir stripes by QoS")
Signed-off-by: Lai Siyao <lai.siyao@whamcloud.com>
Change-Id: I08f94b5b4e355ffff0704bd0f661bb99a82a9234