Expand readonly mode test slightly (#4304)

Must have forgotten to submit the suggestion
for this in #4302

Maintenance is a quick way to check if readonly
mode is working. It doesn't require pulling
any data from graph but will still allow us to
check for a sentinel error.

---

#### Does this PR need a docs update or release note?

- [ ] ✅ Yes, it's included
- [ ] 🕐 Yes, but in a later PR
- [x] ⛔ No

#### Type of change

- [ ] 🌻 Feature
- [ ] 🐛 Bugfix
- [ ] 🗺️ Documentation
- [x] 🤖 Supportability/Tests
- [ ] 💻 CI/Deployment
- [ ] 🧹 Tech Debt/Cleanup

#### Test Plan

- [x] 💪 Manual
- [ ] ⚡ Unit test
- [ ] 💚 E2E

src/pkg/repository/repository_test.go

@@ -6,6 +6,7 @@ import (
 	"time"
 
 	"github.com/alcionai/clues"
+	"github.com/kopia/kopia/repo/blob/readonly"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 	"github.com/stretchr/testify/suite"
@@ -414,8 +415,13 @@ func (suite *RepositoryIntegrationSuite) TestConnect_ReadOnly() {
 	r, err := Connect(ctx, account.Account{}, st, repo.GetID(), control.Options{Repo: ctrlRepo.Options{ReadOnly: true}})
 	assert.NoError(t, err)
 
-	// now we have repoID beforehand
-	assert.Equal(t, r.GetID(), r.GetID())
+	// Maintenance attempts to write some blobs just to say it was running. Since
+	// we're in readonly mode it should fail with a sentinel error.
+	op, err := r.NewMaintenance(ctx, ctrlRepo.Maintenance{})
+	require.NoError(t, err, clues.ToCore(err))
+
+	err = op.Run(ctx)
+	assert.ErrorIs(t, err, readonly.ErrReadonly)
 }
 
 // Test_Options tests that the options are passed through to the repository