fix: create data directory before writing CSVs

[omerbek]

Summary

• create the CSV output parent directory before writing latency results
• return file/write errors from writeToFile instead of exiting inside the helper
• add regression coverage for writing into a missing nested output directory

Fixes #11.

Testing

• git diff --check

Note: Go is not installed in this Windows environment, so go test ./... and gofmt could not be run locally.