tests: add the script that picks the shard durations build

The shards balance against one finished build, so somebody has to choose it.
resolve_shard_durations.py names that build and logs what it holds, for the
trigger job to pass on to every shard.

Resolving here rather than in the shards means an unusable reference stops
the cheap trigger job, which has not built a site yet, rather than the eight
that have.

It takes no view on whether the numbers look right, that differs per suite.
It fails only when the report cannot be read or holds no test cases at all.

CMK-32643

Change-Id: Ifaa68de5429372a2fc1ec091e5e0743a517b9687