test: Clean up preload disabling/enabling

Now that we have commit f0f4976 everywhere (Cockpit 265), we can
use overrides in /etc/cockpit/ instead of mucking around in /usr.

test/common/testlib.py

@@ -1404,19 +1404,15 @@ def is_nondestructive(self):
     def is_devel_build(self):
         return os.environ.get('NODE_ENV') == 'development'
 
-    def disable_preload(self, *packages):
+    def disable_preload(self, *packages, machine=None):
+        if machine is None:
+            machine = self.machine
         for pkg in packages:
-            path = "/usr/share/cockpit/%s" % pkg
-            if self.file_exists(path):
-                if self.machine.ostree_image:
-                    # get a writable directory
-                    self.restore_dir(path)
-                self.write_file("%s/override.json" % path, '{ "preload": [ ] }')
+            machine.write(f"/etc/cockpit/{pkg}.override.json", '{ "preload": [ ] }')
 
     def enable_preload(self, package, *pages):
-        path = "/usr/share/cockpit/%s" % package
-        if self.file_exists(path):
-            self.write_file(path + '/override.json', '{ "preload": [%s]}' % ', '.join('"{0}"'.format(page) for page in pages))
+        pages_str = ', '.join(f'"{page}"' for page in pages)
+        self.machine.write(f"/etc/cockpit/{package}.override.json", f'{{ "preload": [ {pages_str} ] }}')
 
     def system_before(self, version):
         try:

test/verify/check-shell-host-switching

@@ -118,8 +118,7 @@ class TestHostSwitching(MachineCase, HostSwitcherHelpers):
         # In these tests we actually switch between machines in quick succession which can make things even worse
         if self.is_devel_build():
             for machine in ["machine2", "machine3"]:
-                for pkg in ["packagekit", "systemd", "playground"]:
-                    self.machines[machine].write(f"/etc/cockpit/{pkg}.override.json", '{ "preload": [ ] }')
+                self.disable_preload("packagekit", "playground", "systemd", machine=self.machines[machine])
         # Also, quick logouts cause async preloads to run into "ReferenceError: cockpit is not defined"
         self.disable_preload("packagekit", "playground", "systemd")
 

test/verify/check-shell-multi-machine

@@ -168,8 +168,7 @@ class TestMultiMachineAdd(MachineCase):
             # Preloading on machines with debug build can overload the browser and cause slowness and browser crashes
             # In these tests we actually switch between machines in quick succession which can make things even worse
             if self.is_devel_build():
-                m.write("/etc/cockpit/packagekit.override.json", '{ "preload": [ ] }')
-                m.write("/etc/cockpit/systemd.override.json", '{ "preload": [ ] }')
+                self.disable_preload("packagekit", "playground", "systemd")
 
         self.setup_ssh_auth()
 
@@ -277,8 +276,7 @@ class TestMultiMachine(MachineCase):
             # Preloading on machines with debug build can overload the browser and cause slowness and browser crashes
             # In these tests we actually switch between machines in quick succession which can make things even worse
             if self.is_devel_build():
-                m.write("/etc/cockpit/packagekit.override.json", '{ "preload": [ ] }')
-                m.write("/etc/cockpit/systemd.override.json", '{ "preload": [ ] }')
+                self.disable_preload("packagekit", "playground", "systemd")
 
     def checkDirectLogin(self, root='/', known_host=False):
         b = self.browser

test/verify/check-shell-multi-machine-key

@@ -116,8 +116,7 @@ class TestMultiMachineKeyAuth(MachineCase):
         # Preloading on machines with debug build can overload the browser and cause slowness and browser crashes
         # In these tests we actually switch between machines in quick succession which can make things even worse
         if self.is_devel_build():
-            self.machine2.write("/etc/cockpit/packagekit.override.json", '{ "preload": [ ] }')
-            self.machine2.write("/etc/cockpit/systemd.override.json", '{ "preload": [ ] }')
+            self.disable_preload("packagekit", "playground", "systemd", machine=self.machine2)
 
     # Possible workaround - ssh as `admin` and just do `m.execute()`
     @skipBrowser("Firefox cannot do `cockpit.spawn`", "firefox")