test(interfaces): Test for missing ip or ifconfig

We have a fallback to `ifconfig` if `ip` doesn't exist, but it wasn't
tested anywhere that does have `ip` (pretty sure that's all of our current
distro tests).

Introduce a parameterized fixture that "hides" `ip` or `ifconfig` and,
use it in tests.
Also introduce another test that actually makes sure that we find the
correct interfaces.

Follow up to #1090

Author: yedayak

test/t/unit/test_unit_compgen_available_interfaces.py

@@ -1,6 +1,6 @@
 import pytest
 
-from conftest import assert_bash_exec
+from conftest import assert_bash_exec, bash_env_saved
 
 
 @pytest.mark.bashcomp(cmd=None)
@@ -16,10 +16,30 @@ def functions(self, bash):
             '_comp__test_compgen() { local -a arr=(00); _comp_compgen -v arr "$@"; _comp__test_dump; }',
         )
 
-    def test_1_trailing_colons(self, bash, functions):
+    # We fallback to ifconfig if ip fails, so we want to check also the scenario without ip.
+    @pytest.fixture(scope="function", params=["ip", "ifconfig"])
+    def remove_one_tool(self, request, bash):
+        assert_bash_exec(bash, f"{request.param}() {{ false; }}")
+        yield
+        assert_bash_exec(bash, f"unset -f {request.param}")
+
+    def test_1_trailing_colons(self, bash, functions, remove_one_tool):
         output = assert_bash_exec(
             bash,
             "_comp__test_compgen available_interfaces",
             want_output=True,
         )
         assert ":>" not in output.strip()
+
+    def test_2_correct_interfaces(self, bash, functions, remove_one_tool):
+        with bash_env_saved(bash) as bash_env:
+            # Using emulated ip and ifconfig commands
+            bash_env.write_variable(
+                "PATH", "$PWD/shared/bin:$PATH", quote=False
+            )
+            output = assert_bash_exec(
+                bash,
+                "_comp__test_compgen available_interfaces",
+                want_output=True,
+            )
+            assert all(iface in output for iface in ["<eth0>", "<lo>"])