tstest/natlab: add FreeBSD test case for SSH test

Add logic to generateFreeBSDUserData to allow for an SSH connection as
root for FreeBSD.

Add FreeBSD test cases to ssh_test that exercise the same paths as the
existing Ubuntu tests but for BSD.

Updates https://github.com/tailscale/corp/issues/44813
Updates https://github.com/tailscale/tailscale/issues/13038

Signed-off-by: Mario Minardi <mario@tailscale.com>
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