By making the Rails minitest behave like a standard minitest plugin
we're much more likely to not break when people use other minitest
plugins. Like minitest-focus and pride.
To do this, we need to behave like minitest: require files up front
and then perform the plugin behavior via the at_exit hook.
This also saves us a fair bit of wrangling with test file loading.
Finally, since the environment and warnings options have to be applied
as early as possible, and since minitest loads plugins at_exit, they
have to be moved to the test command.
* Don't expect the root method.
It's likely this worked because we eagerly loaded the Rails minitest plugin
and that somehow defined a root method on `Rails`.
* Assign a backtrace to failed exceptions.
Otherwise Minitest pukes when attempting to filter the backtrace (which
Rails' backtrace cleaner then removes).
Means the exception message test has to be revised too.
This is likely caused by the rails minitest plugin now being loaded for
these tests and assigning a default backtrace cleaner.
In the Rails repository we use a `bin/test` executable to run our tests.
However the rerun snippets still included `bin/rails test`:
BEFORE:
```
Failed tests:
bin/rails test test/cases/adapters/postgresql/schema_test.rb:91
```
AFTER:
```
Failed tests:
bin/test test/cases/adapters/postgresql/schema_test.rb:91
```
This adds a script `bin/test` to most Rails framework components. The
script uses the rails minitest plugin to augment the runner.
See https://github.com/rails/rails/pull/19571 for details about the
plugin.
I did not yet add `bin/test` for activerecord, activejob and railties.
These components rely on specific setup performed in the rake-tasks.
