gh-38690: Tweak a few tests to pass when giac is not installed
This is one of the prerequisites to make it possible to install sage
without giac (https://github.com/sagemath/sage/issues/38668). These few
tests can be made to work easily without a `# needs giac`, so let's do
that.
URL: https://github.com/sagemath/sage/pull/38690
Reported by: Michael Orlitzky
Reviewer(s): David Coudert
