test_swap_memory() accepts no free swap

[vstinner]

A system can run with no free swap space.

Summary

• OS: Linux
• Bug fix: yes
• Type: tests
• Fixes: { comma-separated list of issues fixed by this PR, if any }

Description

psutil.tests.test_system.TestMemoryAPIs.test_swap_memory fails if the free swap space is zero. The test makes an assumption which is wrong.

Example when building psutil package on Red Hat Koji build server: https://bugzilla.redhat.com/show_bug.cgi?id=1991086

[vstinner]

I used git commit --amend --no-edit --signoff to sign off my commit.