test/rocksdb: use mkdir -p for results directory

This makes the test script easier to use outside of
the test pool, since it will just reuse the rocksdb
results directory rather than failing because the
directory already exists.

Signed-off-by: Jim Harris <james.r.harris@intel.com>
Change-Id: I754489c3321323bce8c8a5c352d4947d6270d489

Reviewed-on: https://review.gerrithub.io/c/spdk/spdk/+/452260


Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Ben Walker <benjamin.walker@intel.com>
Reviewed-by: Changpeng Liu <changpeng.liu@intel.com>
Reviewed-by: Ziye Yang <ziye.yang@intel.com>
Reviewed-by: Darek Stojaczyk <dariusz.stojaczyk@intel.com>