schemes/elliptic_curves: disable a test that takes too long

[tornaria]

A doctest introduced in #35626 takes ~50s on current hardware (for comparision, doctesting the whole of src/sage/schemes/elliptic_curves/ell_rational_field.py takes ~22s for 889 tests).

This is too long even for --long, but the example is still interesting, so we just mark it # not tested.

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CC: @chriswuthrich (should be a trivial review 🙏)

[chriswuthrich]

Actually, when I implemented this I checked the timings and it wasn't much. If I do ellrank(e) in gp now, it takes 200ms. I believe this could be a bug in the interface with pari rather than a problem with the speed. If so, the test should stay in. I don't know if there were some changes to pari that affect this.

[github-actions]

Documentation preview for this PR (built with commit 902d91c; changes) is ready! 🎉

[tornaria]

Actually, when I implemented this I checked the timings and it wasn't much. If I do ellrank(e) in gp now, it takes 200ms. I believe this could be a bug in the interface with pari rather than a problem with the speed. If so, the test should stay in. I don't know if there were some changes to pari that affect this.

ellrank is quick; what takes time is saturation. Unfortunately E.rank() also calls (unnecessarily?) saturation.

[chriswuthrich]

Yes, it is the saturation. "ellrank" documentation in pari says "... and L is a list of independent, non-torsion rational points on the curve". I don't think it is unnecessary to saturate them.

I am still puzzled as this test used to be quick. I can't see anything that changed to the saturation function directly.

[tornaria]

For generators it seems necessary. For rank, it seems not.

[chriswuthrich]

Even after playing with old version, I can't see how this ran ever faster. So I think it is totally valid to do what this ticket asks. I must misremember things. Sorry.

The particular test is for the function gens, not rank, so the saturation needs to be done. However I spot that line 2227 in this file can be deleted. I am fine doing this later in a different ticket if this needs to go in quickly. Then I will move that example up to rank so that we have a test involving bigints for pari.

[chriswuthrich]

I am happy for this to go in. Positive (trivial) review. Except I forgot what I need to do in this new interface to signal that.