[lldb-dap][test] Migrate server mode tests (#227804)

Migrate TestDAP_server.py to the new lldb-dap test infrastructure.

test_server_port and test_server_unix_socket now run two sessions
concurrently.
Add test_breakpoints_in_multiple_sessions to check that a breakpoint set
in one session must not stop another.
Fix test_connection_timeout_at_server_start. It now waits for lldb-dap
to exit on its own before the teardown kills it.

The test now takes significantly less time.

GitOrigin-RevId: fb45017cd60cf012c85ce4f87498c26e49ca9e5e