sagemathgh-39974: Fix test failure in doctest of rank method
    
Failure seen in
https://github.com/sagemath/sage/pull/39725/files#annotation_33948370801
. It can be computed that the probability of failure is ≈ 1/16007, which
is tiny but still can happen if there is a sufficiently large number of
pull requests the code being ran on.



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URL: sagemath#39974
Reported by: user202729
Reviewer(s):

Author: Release Manager

src/sage/matrix/matrix_modn_dense_template.pxi

@@ -2136,7 +2136,10 @@ cdef class Matrix_modn_dense_template(Matrix_dense):
         ::
 
             sage: # needs sage.rings.finite_rings
-            sage: A = random_matrix(GF(16007), 100, 100)
+            sage: while True:
+            ....:     A = random_matrix(GF(16007), 100, 100)
+            ....:     if A.rank() == 100:
+            ....:         break
             sage: B = copy(A)
             sage: A.rank()
             100