gh-38770: Add "needs" tags for giac and libgiac
Part of https://github.com/sagemath/sage/issues/38668. If it's going to
be possible to disable giac, we need to guard all of the tests that use
it with either `# needs giac` or `# needs sage.libs.giac`.
I think I've gotten them all. A crude way to test:
1. `git rm -r src/sage/libs/giac` and rebuild to disable sage.libs.giac
2. build sage, and then delete the giac executable to disable the
pexpect interface
If you do these one at a time, it should ensure that the correct tags
are used. (Typically, if giac is missing, neither sage.libs.giac nor the
giac executable will be present, making it very easy to mix up the
tags.)
For bonus points you can undelete `src/sage/libs/giac` after building
but before testing to make sure the "needs" tags in those files are
accurate.
### Dependencies:
* https://github.com/sagemath/sage/pull/38756
* https://github.com/sagemath/sage/pull/38686 (not strictly required,
but it adds a few "needs sage.libs.giac" tags of its own)
URL: https://github.com/sagemath/sage/pull/38770
Reported by: Michael Orlitzky
Reviewer(s): Tobias Diez
