WIP - Use flatware to parallelize CI specs

[mjankowski]

Once again comparing codecov<>simplecov integration results for a fresh branch vs the existing - #27663

Ignore this for now.

[mjankowski]

Trying to sort out the codecov/simplecov/flatware formatters/reporting interaction. One theory I had on the other PR is that the LCOV formatter is not properly combining results, but other formatters might. On this branch I'm trying the xml formatter that codecov uses in their examples. I see locally, on this branch:

• Running bin/rspec the report output shows "24859 / 28046 LOC (88.64%) covered"
• Running bin/flatware rspec -r ./spec/flatware_helper.rb also shows "24859 / 28046 LOC (88.64%) covered" which makes it seem like that format is recombined correctly when running in parallel
• Running CI=true bin/rspec shows "25065 / 27769 LOC (90.26%) covered; 5283 / 7674 BC (68.84%) covered"
• Running CI=true bin/flatware rspec -r ./spec/flatware_helper.rb shows "25065 / 27769 LOC (90.26%) covered; 5283 / 7674 BC (68.84%) covered"

For all of these scenarios I see the full 5556 examples all passing.

This makes me think that the re-combination is working correctly, but there's some reporting difference in how simplecov interacts with the default html formatter vs the xml formatter (note the difference in lines covered and total lines, and thus percentage - between the first two examples using html formatter vs last one using xml).

The values reported on CI wind up being slightly different because some of the ENV vars turn on more than the default spec set.