Fix #111: Use correct length filename for too large test

[irowebbn]

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Describe the contribution
Fixes #111

Testing performed
See reproduction steps from linked issue

Expected behavior changes
Tests should pass if OSAL_CONFIG_MAX_PATH_LEN is increased

System(s) tested on

• Hardware: Intel Xeon CPU E5-2687W v4 @ 3.00GHz
• OS: CentOS 7
• Versions: trunk DS, cFE Draco rc-3

Additional context
Add any other context about the contribution here.
Related cFE issue: nasa/cFE#2372

Contributor Info - All information REQUIRED for consideration of pull request
Full name and company/organization/center of all contributors ("Personal" if individual work)
Isaac Rowe, NASA JSC/Jacobs Technology

[dzbaker]

@irowebbn CCB 30 November 2023: Agreed to provisionally approve. A couple of changes are needed:

1. DirName should be declared at top of function.
2. sizeof(char) will always be 1 in C, therefore is not needed.