NIFI-14099 Adjust GetHDFSTest to use TempDir for test files

[exceptionfactory]

Summary

NIFI-14099 Adjusts the behavior of GetHDFSTest to use the JUnit TempDir annotated path for the source of reading files. This approach avoids depending on the timestamp of files located in the source directory itself, making the tests more reliable based on expected creation times.

Additional changes include removing a duplicative test for a directory defined using expression language in the property value.

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  • JDK 21

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[mark-bathori]

Thanks again @exceptionfactory for this improvement, changes looks good now.
LGTM+1
Merging.