[k6] Fix response code check for passthrough write test

The POST operation returns 201, not 200.
This causes checks to fail when running k6 manually.

Issue-ID: CPS-2267
Signed-off-by: danielhanrahan <daniel.hanrahan@est.tech>
Change-Id: I3c0e771b89aa09e41c9dbdf1c532599e1ab5440d
1 file changed