test: grab the SIGNAL_MTX lock in test_sigaction

[SteveLauC]

What does this PR do

This test changes the process-wide signal handler so we should use a lock to serialize it with other tests.

cc @TheJonny

Checklist:

• [x] I have read CONTRIBUTING.md
• [ ] I have written necessary tests and rustdoc comments
• [ ] A change log has been added if this PR modifies nix's API

[SteveLauC]

This issue was found in #2375 by @TheJonny