fix(tests): repair the fixture and doubles that made seven tests fail

Closes netork#110.

The seven failures had two causes, neither of them in the driver.

The `driver` fixture builds a LinuxDriver with `__new__`, bypassing `__init__`,
and never set `_sudo_password`. Every call through `_sudo()` therefore raised
AttributeError, which the callers' broad `except Exception` reported as
`{"success": False}` — so six apply_updates tests failed for a reason unrelated
to what they were asserting.

`test_apply_updates_apt_all_packages` had a second one: its `capture_send(cmd)`
double accepted no keyword arguments, while `_sudo()` passes `read_timeout`.

The seventh, the apt update listing test, supplied `side_effect=["",
APT_UPGRADABLE]` — two values for a cache refresh that never existed.
`_get_updates_apt` has made exactly one `_send` call since it was written in
2712389, so the empty first value was consumed and parsed as the package list.
Checked out that commit and ran it: the test failed there too. It was committed
red and never passed.

That settles the open question in netork#110: the implementation was not changed,
the tests were written against one that never existed. `get_available_updates`
reads the local apt cache deliberately — refreshing it needs sudo and would cost
a round trip on every poll — so there is no stale-cache bug behind the
`updates_available` warning.

tests/test_linux.py

@@ -22,6 +22,11 @@ def driver():
     d._secret = "pass"  # noqa: S105
     d._forced_pkg_manager = None
     d._pkg_manager = "apt"
+    # Set by __init__, which this fixture bypasses via __new__. Without it every
+    # call through _sudo() raises AttributeError, which the callers' broad
+    # `except Exception` turns into a plain {"success": False} -- so the tests
+    # failed for a reason that had nothing to do with what they were testing.
+    d._sudo_password = None
     d.netmiko_optional_args = {}
     d._device = MagicMock()
     return d
@@ -179,7 +184,7 @@ APT_UPGRADABLE = (
 
 def test_get_available_updates_apt(driver):
     driver._pkg_manager = "apt"
-    with patch.object(driver, "_send", side_effect=["", APT_UPGRADABLE]):
+    with patch.object(driver, "_send", return_value=APT_UPGRADABLE):
         updates = driver.get_available_updates()
     assert len(updates) == 2
     assert updates[0]["name"] == "openssh-server"
@@ -312,7 +317,10 @@ def test_apply_updates_apt_all_packages(driver):
     driver._pkg_manager = "apt"
     sent_commands = []
 
-    def capture_send(cmd):
+    def capture_send(cmd, **kwargs):
+        # _sudo() passes read_timeout as a keyword; without **kwargs this raises
+        # TypeError, which the caller's `except Exception` reports as a failed
+        # upgrade rather than a broken test double.
         sent_commands.append(cmd)
         return APT_UPGRADE_SUCCESS