Cache results in _drupalorg_testing_get_tid_by_term()

I noticed this while reviewing the latest patch for #231640: Auto-generate some release nodes:

Neither _drupalorg_testing_get_tid_by_term() nor taxonomy_get_term_by_name() cache their results.

_drupalorg_testing_get_tid_by_term() is called in a nested foreach loop when generating release nodes, and therefore, we're doing A LOT of duplicate queries.

Yes, it's an install profile, so we don't really care about performance that much, but we don't need to ruthlessly pound the DB for nothing. ;) Think of the kittens on shared hosting test sites that might get in trouble with bogus queries/second quotas and the like...