Fix issue with relative paths when loading data streams

[jsoriano]

Relative paths calculation for data streams depends on the paths
having trailing separators or not. Use filepath.Rel to obtain these
relative paths.

[elasticmachine]

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• Start Time: 2021-10-11T11:29:33.146+0000

• Duration: 5 min 35 sec

• Commit: d3341a1

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Test	Results
Failed	0
Passed	114
Skipped	0
Total	114

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