pytest: add tests using sshd

With either /usr/sbin/sshd found or configured via --with-test-sshd=path
add tests for SCP down- and uploads, insecure, with known hosts or not,
with authorized user key or unauthorized one.

Working now with libssh and libssh2, using a hashed known_hosts file.

Closes #19934

docs/INSTALL-CMAKE.md

@@ -483,6 +483,8 @@ Examples:
 - `DANTED`:                                 Default: `danted`
 - `TEST_NGHTTPX`:                           Default: `nghttpx`
 - `VSFTPD`:                                 Default: `vsftps`
+- `SSHD`:                                   Default: `sshd`
+- `SFTPD`:                                  Default: `sftp-server`
 
 ## Feature detection variables