[CELEBORN-1520] Minor logging fix for AppDiskUsageMetric and Fixed UTs

[s0nskar]

What changes were proposed in this pull request?

• Minor logging change in AppDiskUsageMetric which are not critical but slightly bothering.
• Fixed UT's for AppDiskUsageMetric

Why are the changes needed?

1. Current AppDiskUsageMetric UTs were like placeholder and just printing the output. They were not testing/verifying anything.
2. Minor logging change with AppDiskUsageMetric.
   • Comma formating was wrong

Snapshot start 2024-07-24T08:47:12.496 end 2024-07-24T08:57:12.497 Application application_1717149813731_19042841_2 used approximate 15.9 GiB ,Application application_1717149813731_19042841_1 used approximate 13.9 GiB

• We were printing an extra empty line after each summary.

211:20:24.339 [master-app-disk-usage-metrics-logger] INFO  org.apache.celeborn.common.meta.AppDiskUsageMetric - App Disk Usage Top50 Report 
Snapshot start 2024-07-24T09:17:12.498 end 2024-07-24T09:27:12.498 Application application_XXX used approximate 14.5 GiB   
Snapshot start 2024-07-24T08:17:12.495 end 2024-07-24T08:27:12.496 Application application_XXX used approximate 15.9 GiB   

11:27:12.507 [master-app-disk-usage-metrics-logger] INFO  org.apache.celeborn.common.meta.AppDiskUsageMetric - App Disk Usage Top50 Report 
Snapshot start 2024-07-24T09:17:12.498 end 2024-07-24T09:27:12.498 Application application_XXX used approximate 14.5 GiB   
Snapshot start 2024-07-24T08:17:12.495 end 2024-07-24T08:27:12.496 Application application_XXX used approximate 15.9 GiB   

Does this PR introduce any user-facing change?

No

How was this patch tested?

Fixed current UTs and verified from the logs.

[pan3793]

Thanks, merged to main/0.5/0.4

[s0nskar]

Thanks @pan3793 for the review.