Re-enable parallel testing (in development)

This commit re-enables parallel testing. This takes advantage of the
extra cores available on development machines. I've set
`parallelize(workers: 1)` to retain the current behaviour in CI, but
locally a developer can run `PARALLEL_WORKERS=8 bin/rails test` for
example to take advantage of additional cores.

I had to make CreatingAnArtistTest more deterministic - relying on
finding the artist specifically rather than calling `Artist.last` to
make it run alongside other tests in parallel. I also added a text
assertion for good measure to ensure the Artist had been created
before the `artist_path` was visited.

test/system/creating_an_artist_test.rb

@@ -18,8 +18,9 @@ class CreatingAnArtistTest < ApplicationSystemTestCase
     attach_file 'Profile picture', Rails.root.join('test/fixtures/files/cover.png')
 
     click_on 'Save'
+    assert_text 'Artist was successfully created'
 
-    visit artist_path(Artist.last)
+    visit artist_path(Artist.find_by(name: 'The Beatles'))
 
     assert_text 'The Beatles'
     assert_text 'Liverpool'

test/test_helper.rb

@@ -7,6 +7,8 @@
 
 module ActiveSupport
   class TestCase
+    parallelize(workers: 1)
+
     include FactoryBot::Syntax::Methods
 
     WebMock.disable_net_connect!(allow_localhost: true)