[TestSuit] Use consistent assertion for lightClient responses.

[prvyk]

LightClient is used for a lot of the testsuit. It uses a large fixed array, which is trimmed to compare a fixed expected responsed. There are at least 4 separate ways this is done in the testsuit**. Also the comparison is sometimes wrong in the argument order (doesn't use expected response as expected) or very rarely just wrong (CanUseZDiffSTORE test).

This PR introduces a standard TestUtils methods for this assertion, changes almost all relevant locations to use it, and fixes the few cases when it's done incorrectly.

**

1. There's this:
   Encoding.ASCII.GetString(response).Substring(0, expectedResponse.Length).

(Runs GetString + allocation over the entire fixed array).

2. There's also:
   Encoding.ASCII.GetString(response, 0, expectedResponse.Length)

(Reasonable, but allocates. Maybe could be improved)

3. And also:
   ClassicAssert.AreEqual(response.AsSpan().Slice(0, expectedResponse.Length).ToArray(), expectedResponse);

(Order of arguments is wrong. Otherwise, it's interesting to note NUnit can handle the different types, not clear if that is better or worse).

4. Lastly there are calls to lightclient's Execute() method which just does the second under the hood. These aren't modified for now.

This diff uses the Encoding.ASCII.GetString(response, 0, expectedResponse.Length) method. It would be easy the try the latter method (in the correct argument order) if it's useful.

(Re #1063 comments: I don't view this as 'improving lightClient' but as improving the testsuit. If we are stuck with some use for now, might as well test the assertion in a consistent and safe way.)

[badrishc]

(Re #1063 comments: I don't view this as 'improving lightClient' but as improving the testsuit. If we are stuck with some use for now, might as well test the assertion in a consistent and safe way.)

Makes sense. It is worth examining the usage sites of LightClient to make sure it was specifically necessary to use it, versus GCS (GarnetClientSession) or SE.Redis. For example. some tests care about specific network splits which GCS and SE.Redis cannot directly control.