Test that the song cache fills up to the cache size setting

Covers a cache size above 1: songs are preloaded until the setting is
reached, playing a song tops the cache up again, and the cache keeps
filling after a failed download once the server answers again.

tests/fakes/fake_http_transport.gd

@@ -12,6 +12,7 @@ var requested_paths: Array[String] = []
 var requested_methods: Array[int] = []
 
 var _responses: Dictionary = {}
+var _successes_left: Dictionary = {}
 var _previous_transport: HttpTransport
 
 
@@ -39,6 +40,13 @@ func respond_json(path: String, data: Variant, response_code: int = 200) -> void
 ## Answers requests to [param path] with [param body] as-is.
 func respond_bytes(path: String, body: PackedByteArray, response_code: int = 200) -> void:
     _responses[path] = {"code": response_code, "body": body}
+    _successes_left.erase(path)
+
+
+## Lets [param successes] more requests to [param path] succeed, then answers 503 until
+## a new response is registered for the path.
+func fail_after(path: String, successes: int) -> void:
+    _successes_left[path] = successes
 
 
 ## Returns how many times [param path] has been requested.
@@ -50,6 +58,11 @@ func request(url: String, method: int, on_complete: Callable) -> void:
     var path: String = url.trim_prefix(Settings.default_path)
     requested_paths.append(path)
     requested_methods.append(method)
+    if _successes_left.has(path):
+        if _successes_left[path] == 0:
+            on_complete.call(503, PackedByteArray())
+            return
+        _successes_left[path] -= 1
     if _responses.has(path):
         on_complete.call(_responses[path].code, _responses[path].body)
     else: