Unit testing of internal/sshd

Currently internal/sshd has coverage: 10.2% of statements. With !479 (merged) it's coverage: 11.8% of statements

!463 (merged) split the module into 3 files. We can tackle this issue via separate issues:

• For connection.go: #521 (closed)
• For session.go: #522 (closed)
• For sshd.go: #523 (closed)