Test needed for pkg/preflight/concat.go

[diamonwiggins]

Bug Description

There are currently no tests. We should add coverage for a few different scenarios including the bug recently found in https://github.com/replicatedhq/troubleshoot/pull/998

Expected Behavior

Tests are introduced for the preflight concat code and they are passing

Additional Context

Include the following information.

• Troubleshoot version. If you built from source, note that including the version of Go you used to build with.
• Operating system
• Operating system version
• Other details that might be helpful in diagnosing the problem