Use a larger data chunk to test compression (#4115)

Test that compression is on by default by having one file be large and easily compressible. Compression is indirectly checked by looking at how many bytes were uploaded.

---

#### 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
- [x] 🧹 Tech Debt/Cleanup

#### Issue(s)

* closes #3156

#### Test Plan

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

src/internal/kopia/wrapper_test.go

@@ -5,6 +5,7 @@ import (
 	"context"
 	"io"
 	stdpath "path"
+	"strings"
 	"testing"
 	"time"
 
@@ -765,18 +766,24 @@ func (suite *KopiaIntegrationSuite) TearDownTest() {
 }
 
 func (suite *KopiaIntegrationSuite) TestBackupCollections() {
-	collections := []data.BackupCollection{
-		exchMock.NewCollection(
+	c1 := exchMock.NewCollection(
 		suite.storePath1,
 		suite.locPath1,
-			5),
+		5)
+	// Add a 4k chunk of data that should be compressible. This helps check
+	// compression is enabled because we do some testing on the number of bytes
+	// uploaded during the first backup.
+	c1.Data[0] = []byte(strings.Repeat("abcdefgh", 512))
+
+	collections := []data.BackupCollection{
+		c1,
 		exchMock.NewCollection(
 			suite.storePath2,
 			suite.locPath2,
 			42),
 	}
 
-	c1 := exchMock.NewCollection(
+	c1 = exchMock.NewCollection(
 		suite.storePath1,
 		suite.locPath1,
 		0)