Fix rspec-rails incompatbility with Rails 7

rspec-rails v6.0.0.rc1 needs to be used to support `have_enqueued_email`
in Rails 7 (see rspec/rspec-rails#2546).
However, that would break tests for previous versions. On top of that,
`rspec-rails` dependency is transitive through `solidus_dev_support`,
which is not compatible with rsec-rails v6.0.0.rc1.

spec/models/spree/order_spec.rb

@@ -45,19 +45,19 @@
       it 'does not send the confirmation email' do
         order = Spree::TestingSupport::OrderWalkthrough.up_to(:payment)
 
-        expect {
-          order.complete!
-        }.not_to have_enqueued_email(Spree::OrderMailer, 'confirm_email')
+        expect(Spree::OrderMailer).not_to receive(:confirm_email)
+
+        order.complete!
       end
     end
 
     context 'when disable_builtin_emails is false', unless: ENV['DISABLE_BUILTIN_EMAILS'] do
       it 'sends the confirmation email' do
         order = Spree::TestingSupport::OrderWalkthrough.up_to(:payment)
 
-        expect {
-          order.complete!
-        }.to have_enqueued_email(Spree::OrderMailer, 'confirm_email')
+        expect(Spree::OrderMailer).to receive(:confirm_email).and_call_original
+
+        order.complete!
       end
     end
   end
@@ -79,9 +79,9 @@
         allow(SolidusTracking.configuration).to receive(:disable_builtin_emails).and_return(true)
         order = create(:completed_order_with_totals)
 
-        expect {
-          order.canceled_by(create(:user))
-        }.not_to have_enqueued_email(Spree::OrderMailer, 'cancel_email')
+        expect(Spree::OrderMailer).not_to receive(:cancel_email)
+
+        order.canceled_by(create(:user))
       end
     end
 
@@ -90,9 +90,9 @@
         allow(SolidusTracking.configuration).to receive(:disable_builtin_emails).and_return(false)
         order = create(:completed_order_with_totals)
 
-        expect {
-          order.canceled_by(create(:user))
-        }.to have_enqueued_email(Spree::OrderMailer, 'cancel_email')
+        expect(Spree::OrderMailer).not_to receive(:cancel_email).and_call_original
+
+        order.canceled_by(create(:user))
       end
     end
   end