Use mock module and move tests

[DominicOram]

Fixes #45

To test:

• Confirm tests still pass despite mock and test being moved out of top level

[joeshannon]

Ideally the tests would be separate from the main module(s) so that they wouldn't be present in the application.
Something like a new src directory containing the diffcalc module (and the other modules) and then leaving the tests where they are?
It looks like it would be more work as would have to check the structure still works and might have other implications for development (installing package in dev mode etc.)
Also the GDA diffcalc ScriptProject configuration would be updated.

Due to these complications maybe it is easier to move the tests as you have done but I might ask for a second opinion.

I think it might also be good to wait until we can get the CI running again (#46) to verify the tests can still all run as expected.

[joeshannon]

@isikhar do you have any objections to this being merged?
We can look at making a nicer/more standard solution later possibly but might require more involved changes to GDA.

I would still like to check that we can still run the tests before merging.

[joeshannon]

We should merge #46 first, then fix-up this so that the tests pass.

[DominicOram]

@joeshannon and @isikhar, this ok?

[joeshannon]

Looks good to me. I think Irakli was previously happy with these changes.

[isikhar]

These changes are fine with me as well.

[DominicOram]

Is the CI known to be broken?

[joeshannon]

No I don't think so. Not sure why it's not running. I see that it run for your fork.

I'm not hugely familiar with GH actions, we currently have

name: Diffcalc build

on:
  push:
  pull_request:

Does that seem correct?

[DominicOram]

So it did (see https://github.com/DominicOram/diffcalc/actions/runs/3338486618/jobs/5526204223). Yes, those actions seem correct to me. Are we happy to chalk it up to GH weirdness and get I'll merge this?