gh-35513: Silence initialization of giac
When sage.libs.giac.giac is imported for the first time, some noise is
output to stderr. This is annoying and complicates doctesting as `...`
has to be added to the _first_ time giac is used.
This output comes from the line
Pygen('add_language(1)').eval() # Add the french keywords in the
giac library language.
which doesn't seem to be necessary.
This commit removes the line, and fixes a few doctests which where
expecting some output when using libgiac for the first time.
Also label `# long time` a doctest in `src/sage/calculus/functional.py`
which takes 6-7 seconds. Doing this in the first place motivates the
current change as the initialization noise would happen in different
tests depending on whether `--long` or not is used.
Same for a doctest in `src/sage/calculus/tests.py`, and one in
`src/sage/symbolic/integration/integral.py`. A test in
`src/sage/libs/giac/giac.pyx` was marked `# random` but after this fix
it seems to work ok.
### :memo: Checklist
- [x] The title is concise, informative, and self-explanatory.
- [x] The description explains in detail what this PR is about.
- [x] There are existing tests covering the changes.
URL: https://github.com/sagemath/sage/pull/35513
Reported by: Gonzalo TornarÃa
Reviewer(s): François Bissey
