tls: Add unit test for partial writes
tls: Add unit test for partial writes

Set socket buffer of the ws receiving end to a small value and try to
write a large block, so that `connection_write()` runs into a `PARTIAL`
write.

Closes #12708

moraleslazaro/cockpit,cockpituous/cockpit,martinpitt/cockpit,garrett/cockpit,moraleslazaro/cockpit,cockpit-project/cockpit,cockpit-project/cockpit,mvollmer/cockpit,cockpit-project/cockpit,cockpituous/cockpit,martinpitt/cockpit,mvollmer/cockpit,cockpituous/cockpit,garrett/cockpit,cockpit-project/cockpit,cockpituous/cockpit,moraleslazaro/cockpit,deryni/cockpit,mvollmer/cockpit,cockpituous/cockpit,garrett/cockpit,moraleslazaro/cockpit,garrett/cockpit,moraleslazaro/cockpit,deryni/cockpit,martinpitt/cockpit,moraleslazaro/cockpit,deryni/cockpit,garrett/cockpit,deryni/cockpit,mvollmer/cockpit,moraleslazaro/cockpit,mvollmer/cockpit,martinpitt/cockpit,deryni/cockpit,martinpitt/cockpit,cockpit-project/cockpit,deryni/cockpit,deryni/cockpit,cockpituous/cockpit