nixosTests.wiki-js: fix test on i686-linux

Fail pattern:
1. Unsuspecting `qemu-kvm` notice:
```
server # qemu-kvm: at most 2047 MB RAM can be simulated
```

2. Hard fail
```
    self.shell.send(out_command.encode())
BrokenPipeError: [Errno 32] Broken pipe
```

(Took me a while to consider those lines are related)

nixos/tests/wiki-js.nix

@@ -5,7 +5,7 @@ import ./make-test-python.nix ({ pkgs, lib, ...} : {
   };
 
   nodes.machine = { pkgs, ... }: {
-    virtualisation.memorySize = 2048;
+    virtualisation.memorySize = 2047;
     services.wiki-js = {
       enable = true;
       settings.db.host = "/run/postgresql";