feat: say whether a host is still in its first boot

first_boot_pending() is true while /firstboot exists on FreeBSD. A FreeBSD
cloud image upgrades its base system on its first boot, starts sshd only
after that and restarts right away, and /etc/rc removes /firstboot just
before that restart. netOrk waits for this before it sets up a new VM
(NetOrk/netork#795).

OpenBSD has no such marker to ask yet, so it returns False there.

tests/test_drivers.py

@@ -610,3 +610,26 @@ class TestAFailedReadRaises:
         driver = _channel(OpenBSDDriver, answers)
         with pytest.raises(RuntimeError, match=failing.split()[0]):
             driver.get_available_updates()
+
+
+class TestFirstBoot:
+    """Whether the host is still in its first boot (NetOrk/netork#795).
+
+    A FreeBSD cloud image upgrades its base system on the first boot, starts
+    sshd only then and restarts right after; /etc/rc removes /firstboot just
+    before that restart. netOrk waits for it before setting up a new VM.
+    """
+
+    def test_freebsd_in_its_first_boot(self):
+        driver = _channel(FreeBSDDriver, {"test -e /firstboot": ("", 0)})
+        assert driver.first_boot_pending() is True
+        assert driver.calls == [("test -e /firstboot", False)]
+
+    def test_freebsd_after_its_first_boot(self):
+        driver = _channel(FreeBSDDriver, {"test -e /firstboot": ("", 1)})
+        assert driver.first_boot_pending() is False
+
+    def test_openbsd_has_no_marker_to_ask(self):
+        driver = _channel(OpenBSDDriver, {})
+        assert driver.first_boot_pending() is False
+        assert driver.calls == []