tests: fix json syntax error

When detsys-specific items were removed, some lines of JSON were
removed. This caused syntax errors due to the trailing commas.

Also use this opportunity to add `enable_flakes`, which is required
in new lix-installer plan files

Change-Id: I7874b3b19cb2e15f82b0733894c52b3a23c0cbe5

tests/fixtures/linux/linux.json

@@ -416,6 +416,7 @@
       "ssl_cert_file": null,
       "extra_conf": [],
       "force": false,
+      "enable_flakes": true
     },
     "init": {
       "init": "Systemd",

tests/fixtures/linux/steam-deck.json

@@ -400,6 +400,7 @@
       "ssl_cert_file": null,
       "extra_conf": [],
       "force": false,
+      "enable_flakes": true
     }
   },
   "diagnostic_data": {

tests/fixtures/macos/macos.json

@@ -427,6 +427,7 @@
       "ssl_cert_file": null,
       "extra_conf": [],
       "force": false,
+      "enable_flakes": true
     },
     "encrypt": null,
     "case_sensitive": false,