ut/blob: simplify blob creation and opening

Operation of creating a blob (with or w/o opts),
then opening it through out the tests is quite common.

This patch creates helper function ut_blob_create_and_open(),
to do exactly that with the right asserts.
Throughout the UT this function replaces all instances where
blob is created and opened, when it is not the subject of
particular unit test (focus is on other functionality).

It will allow to later use it in setup/cleanup functions
for UT suite - similar to bs init.

Signed-off-by: Tomasz Zawadzki <tomasz.zawadzki@intel.com>
Change-Id: Ia541b855d154503f824bfa3909bcfeac649c5853
Reviewed-on: https://review.spdk.io/gerrit/c/spdk/spdk/+/1201


Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Ben Walker <benjamin.walker@intel.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>
Reviewed-by: Paul Luse <paul.e.luse@intel.com>