PR summary

This PR unbreaks CI by modifying the failing test.

Since the X and Y shape match the Z shape, they represent the centres of the boxes.pcolor therefore needs to work out the locations of the box corners. In this case, every other column of X and Y were masked, sopcolor can't reasonably be expected to do that.

It previously passed because our use ofnp.hstack stripped the mask off, so it was the same as if we'd not added the mask to begin with. For the test data, there were sensible numbers under the mask but this would obviously not be true in general. We now (since#25027) usenp.ma.hstack.

The point of the test is to verify that we don't attempt to write anything back to the input mask (#26230) . It doesn't matter for those purposes which elements areTrue orFalse.

PR checklist

• "closes #0000" is in the body of the PR description tolink the related issue
• new and changed code istested
• Plotting related features are demonstrated in anexample
• New Features andAPI Changes are noted with adirective and release note
• Documentation complies withgeneral anddocstring guidelines