Update aggregation performance tests

[danielkza]

• Allow more customization in the test runner by setting the measure mode and number of runs through the environment (using the PROFILE_NUM_RUNS and RUBY_PROF_MEASURE_MODE vars - see the RubyProf documentation for the latter).
• Save reports in ./prof/{test_name} by default
• Print current state during tests instead of all at the end. Makes it easier to verify what's currently happening, such as whether we are on setup, teardown, or which run
• Insert TreeMetricResults in bulk, using the activerecord-import gem

[rafamanzo]

Nice job. Just keep in mind that atomic commits make the review and future modifications easier, please give an special attention to this in the next PRs.

:trophy:

[danielkza]

You're right, it was an oversight of mine. Since I had so much code to commit I only ended up splitting up the other branches and not this one. I'll be more careful with the rest!