Revert "tests/requirements.txt: bump up avocado-framework version to 101.0"

Revert "tests/requirements.txt: bump up avocado-framework version to 101.0"

This reverts commit ec5ffa0056389c3c10ea2de1e78366f66f4e5abc.

Bumping avocado to version 101 has two issues.  First, there are problems
where Avocado is not logging of command lines or terminal output, and not
collecting Python logs outside the avocado namespace.

Second, the recent changes to Python handling mean that there is a single
virtual environment for all the build, instead of a separate one for testing.
Requiring a too-new version of avocado causes conflicts with any avocado
plugins installed on the host:

   $ make check-venv
   make[1]: Entering directory '/home/berrange/src/virt/qemu/build'
     GIT     ui/keycodemapdb tests/fp/berkeley-testfloat-3 tests/fp/berkeley-softfloat-3 dtc
     VENVPIP install -e /home/berrange/src/virt/qemu/python/
     VENVPIP install -r /home/berrange/src/virt/qemu/tests/requirements.txt
   ERROR: pip's dependency resolver does not currently take into account all the packages that are installed. This behaviour is the source of the following dependency conflicts.
   avocado-framework-plugin-varianter-yaml-to-mux 98.0 requires avocado-framework==98.0, but you have avocado-framework 101.0 which is incompatible.
   avocado-framework-plugin-result-html 98.0 requires avocado-framework==98.0, but you have avocado-framework 101.0 which is incompatible.
   make[1]: Leaving directory '/home/berrange/src/virt/qemu/build'

To avoid this issue, tests/requirements.txt should use a ">=" constraint
and the version of Avocado should be limited to what distros provide
in the system packages.  Only Fedora has Avocado, and more specifically
version 92.0 (though 98.0 is also available as a module).  As a first
step, this patch reverts the introduction of a too-new Avocado.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>