testing: make unit test expectations portable

[massimomazzariol]

Summary

• Build expected test paths with filepath.Join instead of hard-coded / separators.
• Replace platform-specific error-message regexes with structured error checks using errors.Is and errors.As.
• Normalize CRLF line endings before comparing appended configuration content.
• Add a Changie entry for the fix.

Closes #673.

Testing

The affected packages were tested locally on Windows 11 with both supported Go versions:

go1.25.12: go test ./config ./internal/teststep -count=1
go1.26.5:  go test ./config ./internal/teststep -count=1

Both passed.

Linux behavior was also validated from the Windows development machine using Docker. The same package tests were run from a Linux filesystem checkout inside the official golang:1.25.12 and golang:1.26.5 images, rather than directly against the Windows bind mount, so Linux file permissions and path behavior were exercised normally. Both container runs passed.