apps: Fix reading AppStream metadata config

Commit 7bbf916 introduced a major bug: If the OS in os-release was
not found in the iteration, get_config() would return the whole map
instead of the default value. That crashed the page with
`TypeError: names.forEach is not a function`.

This is awkward to do correctly with .find(), especially due to
modifying `val` as a side effect; iterate over `os_list` the good old
`for` way instead.

This was hidden by the test's manifest override leaving
`appstream_config_packages` intact, so it was always taken from the real
underlying OS. Override it as well.

Cherry-picked from 0eeb475

pkg/apps/application-list.jsx

@@ -103,15 +103,13 @@ export const ApplicationList = ({ metainfo_db, appProgress, appProgressTitle, ac
         const os_list = [os_release.ID || "", ...(os_release.ID_LIKE || "").split(/\s+/)];
 
         if (cockpit.manifests.apps && cockpit.manifests.apps.config) {
-            let val = cockpit.manifests.apps.config[name];
+            const val = cockpit.manifests.apps.config[name];
             if (typeof val === 'object' && val !== null && !Array.isArray(val)) {
-                os_list.find(c => {
-                    if (val[c]) {
-                        val = val[c];
-                        return true;
-                    }
-                    return false;
-                });
+                for (const os of os_list) {
+                    if (val[os])
+                        return val[os];
+                }
+                return def;
             }
             return val !== undefined ? val : def;
         } else {

test/verify/check-apps

@@ -147,7 +147,8 @@ class TestApps(packagelib.PackageCase):
         # use a fake distro map
         self.write_file("/etc/cockpit/apps.override.json",
                         '{ "config": { "appstream_data_packages":'
-                        '    {"testy": ["appstream-data-test"], "otheros": ["nosuchpackage"]}'
+                        '    {"testy": ["appstream-data-test"], "otheros": ["nosuchpackage"]},'
+                        '              "appstream_config_packages": []'
                         '  }}')
 
         self.createAppStreamPackage("app-1", "1.0", "1")