Update test output for compatibility with pluralisation changes in GAP

[wilfwilson]

I've made a GAP pull request at https://github.com/gap-system/gap/pull/4343 which makes a few pluralisation-related changes. The change most relevant to this issue is that it changes “1 arguments” to “1 argument” in no method found errors. I'm expecting this will be merged into GAP master soon, and this PR assumes that it'll make its way into GAP 4.16.

I think the only ways to accommodate this change in our test files would either be to stop testing for no method found errors (which I think would be a reasonable approach; why test for the non-existence of a method?), or to use the #@if syntax and provide two versions of test output, using the appropriate pluralisation for the GAP version. That's what this pull request does.

Unfortunately, the #@if syntax was only introduced in GAP 4.11, so merging this PR will also require dropping support for GAP 4.10; see #866.

There'll be a bit of a chicken-and-egg problem with these pull requests; in particular, the tests here against the GAP master branch will fail until https://github.com/gap-system/gap/pull/4343 is merged.

However, to mitigate this, I've (temporarily) added a CI job to this pull request that tests against the update-pluralization branch of my GAP work, which is the one responsible for https://github.com/gap-system/gap/pull/4343. Therefore these jobs reflect how the GAP master jobs will behave once https://github.com/gap-system/gap/pull/4343 is merged, and therefore these new jobs should pass!

The Semigroups and smallsemi packages also need such an update; I'll make pull requests for all three, and try and merge all those of those, plus https://github.com/gap-system/gap/pull/4343, pretty much simultaneously, so that the period of time during which the CI fails is minimised.