Fix testsuite with singlethreaded

[embhorn]

Fixes issues when wolfSSL is configured with --enable-singletthreaded Also added workflow to test single thread build.

Reproduce with

wolfSSL

./configure --enable-wolfssh  --enable-singlethreaded --enable-keygen

wolfSSH

./configure --enable-all

Fixes issue reported in https://www.wolfssl.com/forums/topic2133-how-to-compile-wolfssh-without-multithreading.html