Rename "client unit tests" to something more appropriate

[sodic]

As mentioned in #2049, back when Wasp treated client and server code differently (pre v0.12.0), we implemented a framework for running client tests using Vitest.

This framework can now be used to test any JS code (client or server), so we should:

• Understand the possibilities and limitations of our "client test" framework.
• Rephrase the docs to communicate that the testing is no longer limited to the client.

A user said:

From my perspective, the thing I found confusing about that “client unit test” example is that it’s not necessarily bound to client code. You can test any javascript code, whether it runs on the client, server, or both!

[Martinsos]

To a degree this is a duplicate of #2049