Fix thread safety issues in pty and termios tests

[asomers]

ptsname(3) returns a pointer to a global variable, so it isn't
thread-safe. Protect it with a mutex.

[Susurrus]

Have you seen this be an actual issue? My understanding is that this is just like the errno strings, where the strings are static in global memory, but they never change, so you should be able to pass around pointers to them willy-nilly.

[asomers]

Yep. If I run the tests with --test-threads=8, I get failures due to ptsname within seconds. This is the very reason that ptsname_r exists. It's like the difference between strerror and strerror_r

[Susurrus]

Hmm, I've never seen those fail locally, but this does seem like the right fix.

LGTM

bors r+

[asomers]

Since bors is stuck, I'm going to manually merge

bors r-