solved test issues with k3d in test_field.py, test_mesh.py, test_region

[AdamsMP92]

## Summary

This PR updates the `k3d`-related tests to avoid calling `k3d.Plot.display()` during pytest runs.

Instead of relying on the plotting methods to create and display a `k3d` plot internally, the tests now create an explicit `k3d.plot()` object and pass it via the `plot` argument.

## Motivation

With `k3d==2.17.0`, calling

```python
k3d.plot().display()

fails in a non-IPython pytest session with

NameError: name 'display' is not defined

because k3d.plot.plot_display calls display(...) without importing it from IPython.display.

The purpose of these tests is to exercise the discretisedfield k3d plotting code paths, not to test whether the interactive Jupyter display machinery is available in a terminal pytest environment.

Changes

• Pass explicit k3d.plot() objects to k3d plotting calls in tests.
• Avoid direct calls to plot.display() in tests.
• Keep the existing k3d plotting paths covered for Field, Mesh, and subregion plotting.

Test result

The local test suite now passes:

3636 passed, 2 skipped, 5 xfailed in 40.11s

[github-actions]

Failed to generate code suggestions for PR

[lang-m]

The reason for the failure is a bug in k3d 2.17. #544 excludes that problematic version.