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.
The errors message only was not displayed, as if it did not use the inline reporting,
modified to also information the method name and the like in error are displayed.
```
# before
Failed assertion, no message given.
bin/rails test test/models/user_test.rb:5
```
```
# after
Failure:
UserTest#test_the_truth:
Failed assertion, no message given.
bin/rails test test/models/user_test.rb:5
```
Any failures or errors will be reported inline during the run by default.
Skipped tests will be reported if run in verbose mode.
Any result is output with failure messages and a rerun snippet for that test.
Rerun snippets won't be output after a run, unless `--defer-output` is passed.
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
```
