networkd-test: change test_transient_hostname_with_static to use an explicit static hostname

Previously, the test would use the existing static hostname. However,
this woud not work as expected in the static hostname was "localhost"
because the transient hostname will override the static one in that case
anyway, as the assumption hostnamed makes is that "localhost" is a
non-initialized hostname.

Hence when testing this, let's first set the static hostname to
something specific first (that is not "localhost").

test/networkd-test.py

@@ -742,8 +742,16 @@ Domains= ~company ~lab''')
 
         orig_hostname = socket.gethostname()
         self.addCleanup(socket.sethostname, orig_hostname)
+
         if not os.path.exists('/etc/hostname'):
-            self.write_config('/etc/hostname', orig_hostname)
+            self.write_config('/etc/hostname', "foobarqux")
+        else:
+            self.write_config('/run/hostname.tmp', "foobarqux")
+            subprocess.check_call(['mount', '--bind', '/run/hostname.tmp', '/etc/hostname'])
+            self.addCleanup(subprocess.call, ['umount', '/etc/hostname'])
+
+        socket.sethostname("foobarqux");
+
         subprocess.check_call(['systemctl', 'stop', 'systemd-hostnamed.service'])
         self.addCleanup(subprocess.call, ['systemctl', 'stop', 'systemd-hostnamed.service'])
 
@@ -755,7 +763,7 @@ Domains= ~company ~lab''')
             out = subprocess.check_output(['ip', '-4', 'a', 'show', 'dev', self.iface])
             self.assertRegex(out, b'inet 192.168.5.210/24 .* scope global dynamic')
             # static hostname wins over transient one, thus *not* applied
-            self.assertEqual(socket.gethostname(), orig_hostname)
+            self.assertEqual(socket.gethostname(), "foobarqux")
         except AssertionError:
             self.show_journal('systemd-networkd.service')
             self.show_journal('systemd-hostnamed.service')