Prevent Rails.env change from leaking to other tests

How to reproduce the flaky failure caused by the leak:

```sh-session
bin/test --seed=1 -v test/generators/scaffold_generator_test.rb:517 test/commands/console_test.rb:51

Run options: --seed=1 -v

Rails::ConsoleTest#test_console_with_environment = 0.00 s = .
ScaffoldGeneratorTest#test_scaffold_generator_database_with_aliases =
0.13 s = F

Failure:
ScaffoldGeneratorTest#test_scaffold_generator_database_with_aliases
[test/generators/scaffold_generator_test.rb:521]:
Expected migration db/secondary_migrate/create_posts.rb to exist, but
was not found
```

It looks like an instance of `Rails::Console#start` will set `Rails.env`
if the console was initialized with an environment option:

https://github.com/rails/rails/blob/fac6689b41a7086ac2dee06770b491bf8b2864c1/railties/lib/rails/commands/console/console_command.rb#L40-L50

`Rails.env` is not reset when the test finishes, which can sometimes
influence (and break other tests).

This fix adds a setup/teardown to the offending test file to mitigate
the leak.

I'm starting to wonder if we need a more nuclear approach... Should we
have a global setup/teardown that nukes and restores the `Rails`
constant completely? (eg. restore a duplicate)

Author: davidstosik

railties/test/commands/console_test.rb

@@ -18,6 +18,14 @@ def self.start
     end
   end
 
+  def setup
+    @prev_rails_env = Rails.env
+  end
+
+  def teardown
+    Rails.env = @prev_rails_env
+  end
+
   def test_sandbox_option
     console = Rails::Console.new(app, parse_arguments(["--sandbox"]))
     assert_predicate console, :sandbox?