add doctest to charpoly method of symbolic matrices

[tmonteil]

As reported on ​this ask question, the following gives wrong result on Archlinux. Though this distro is not officially supported, this example shows some unexpected behaviour that might appear elsewhere, hence it makes sense to add it on the Sage doctests.

sage: M = matrix(SR, 7, 7, SR.var('a', 49))
sage: M.charpoly().degree()
7

[tmonteil]

New commits:

​9000bf5

#26427 : add doctest for charpoly of matrices over symbolic ring

[arojas]

FTR, the incorrect result was caused by building pynac with giac support.

[chapoton]

should be tagged "long" ?

Warning, slow doctest:
    M.charpoly().degree()
Test ran for 1.53 s

[git]

Branch pushed to git repo; I updated commit sha1. New commits:

​1332156

#26427 : log time doctest

[git]

Branch pushed to git repo; I updated commit sha1. This was a forced push. New commits:

​52e1dc2

#26427 : long time doctest

[tmonteil]

You are right, thanks for noticing, i updated the doctest.

[chapoton]

ok

[embray]

This should be re-targeted for 8.5.