src/sage/graphs/generic_graph.py: work around doctest hang

One doctest in this file is "hanging" on ARM64 and RISC-V as GLPK
tries courageously to solve a MIP. A tweak to the solver options
allows this problem to be solved on those two architectures without
affecting any others. This is unlikely to solve the general problem,
but it may buy us some time.

Closes #34575
Closes #38831

src/sage/graphs/generic_graph.py

@@ -7358,6 +7358,24 @@ def edge_disjoint_spanning_trees(self, k, algorithm=None, root=None, solver=None
                 p.add_constraint(pos[root, c] + BFS[u] <= pos[u, c])
 
         # We now solve this program and extract the solution
+        try:
+            # The MIP approach with GLPK is prone to compiler and
+            # optimization-level weirdness on some hardware:
+            #
+            #   * https://github.com/sagemath/sage/issues/34575
+            #   * https://github.com/sagemath/sage/issues/38831
+            #
+            # Disabling the presolver manages to perturb reality just
+            # enough in the one scenario that we doctest explicitly to
+            # "fix" the problem. It's also limited enough in scope
+            # that it probably hasn't badly broken some other use
+            # case.
+            p.solver_parameter("presolve_intopt", False)
+        except KeyError, NotImplementedError, ValueError:
+            # Must not be using GLPK. The specific exception raised
+            # depends on the backend; note that several of them live
+            # outside of the main sagelib repository.
+            pass
         try:
             p.solve(log=verbose)
         except MIPSolverException: