Parallelize test suite

In the past, the test suite has been flaky, and didn't seem to truncate
tables from previous runs.

The root cause of that might be related to [rails/rails#46820][]. In the
meantime, while that is resolved, this commit adds a line to
[parallelize][] to the test suite, and some corresponding `.sqlite3`
ignore values to the `.gitignore`.

[rails/rails#46820]: rails/rails#46820
[parallelize]: https://edgeapi.rubyonrails.org/classes/ActiveSupport/TestCase.html#method-c-parallelize

.gitignore

@@ -1,3 +1,4 @@
 *.gem
 /node_modules
 Gemfile.lock
+*.sqlite3*

test/test_helper.rb

@@ -15,6 +15,8 @@ def render(...)
 
 class ActiveSupport::TestCase
   include ActiveJob::TestHelper
+
+  parallelize(workers: :number_of_processors, threshold: 2)
 end
 
 class ActionDispatch::IntegrationTest