gh-39567: Make test deterministic to fix CI
    
Apparently the test is introduced in
#38824 and  it's basically copied
from another test below that is marked `random`

```
            sage: E = EllipticCurve([-127^2,0])
            sage: E.gens(use_database=False, algorithm='pari',
pari_effort=4)   # long time, random
            [(611429153205013185025/9492121848205441 :
15118836457596902442737698070880/924793900700594415341761 : 1)]
```

I don't know why this one isn't marked `# random` but I choose to keep
checking the output but does it more carefully. Basically it has rank 1
so the generator is determined up to `± 1` and modulo torsion, so we
just need to check `l[0] - a` or `l[0] + a` is in torsion.

It's not clear why the test starts failing now. Maybe it's just random
after all.

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URL: #39567
Reported by: user202729
Reviewer(s): Frédéric Chapoton

Author: Release Manager

src/sage/schemes/elliptic_curves/ell_rational_field.py

@@ -2354,12 +2354,14 @@ def gens(self, proof=None, **kwds):
             sage: set(E.gens()) <= set([P,-P])
             True
 
-        Check that :issue:`38813` has been fixed:
+        Check that :issue:`38813` has been fixed::
 
-            sage: set_random_seed(91390048253425197917505296851335255685)
+            sage: # long time
             sage: E = EllipticCurve([-127^2,0])
-            sage: E.gens(use_database=False, algorithm='pari', pari_effort=4)   # long time
+            sage: l = E.gens(use_database=False, algorithm='pari', pari_effort=4); l   # random
             [(611429153205013185025/9492121848205441 : 15118836457596902442737698070880/924793900700594415341761 : 1)]
+            sage: a = E(611429153205013185025/9492121848205441, 15118836457596902442737698070880/924793900700594415341761)
+            sage: assert len(l) == 1 and ((l[0] - a).is_finite_order() or (l[0] + a).is_finite_order())
         """
         if proof is None:
             from sage.structure.proof.proof import get_flag