[dynamo][easy] Only xfail test_dynamic_shapes_float_guard_dynamic_shapes if z3 is available (#107137)
This test only fails when z3 is available. So we should only xfail it if z3 is available, otherwise the test passes with an unexpected success.
Differential Revision: [D48323103](https://our.internmc.facebook.com/intern/diff/D48323103)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/107137
Approved by: https://github.com/ysiraichi, https://github.com/williamwen42
