chore: remove empty testutils package (open-feature#55)

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## This PR
Removes empty testutils package

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Signed-off-by: Florin-Mihai Anghel <fanghel@google.com>
Signed-off-by: Florin-Mihai Anghel <44744433+anghelflorinm@users.noreply.github.com>
Co-authored-by: Michael Beemer <beeme1mr@users.noreply.github.com>

Author: anghelflorinm