[k6] Use constant arrival rate

[k6] Use constant arrival rate

Currently, k6 KPI tests send requests using 'constant-vus' executor.
This continuously sends requests on different threads - as soon as
one request completes, another is sent. This leads to weird results
where improving performance of one operation ends up degrading other
test cases, due to more requests being sent per second.

This change makes KPI tests send requests at a constant rate,
making the mixed-load tests less prone to such anomalies.

- Send requests at constant rates for test cases 3, 4, 5, 6.
- Throughput tests 1, 2 & 7 still send at the fastest rate.
- Adjust expected performance timings.

Issue-ID: CPS-2519
Signed-off-by: danielhanrahan <daniel.hanrahan@est.tech>
Change-Id: I641ead7d2d9d3b29116e1cf0d4bf8cd345b01a98