Load settings automatically for each test

This uses a fixture to load the default settings file before each test.
Tests that require a different settings file (ie test_settings) can be
configured to skip this step using this line of code:

    @pytest.mark.load_default_settings(False)

This change is necessary to be able to run these tests in isolation from
each other. The `altlink`, `getUrl` and `downloadBinaryFile` functions
are all dependent on Settings, and you cannot test them without loading
a Settings file first. Without this change, when these tests ran, they
used a Settings object loaded up in some previous test, which could have
loaded some Settings file that we do not want. If we try to run these
tests without loading a Settings file, they will just fail to run
because Settings doesn't exist yet.

tests/test_helper.py

@@ -14,6 +14,13 @@ from aqt.helper import Settings, get_hash, getUrl, retry_on_errors
 from aqt.metadata import Version
 
 
+@pytest.fixture(autouse=True)
+def load_default_settings(use_defaults: bool = True):
+    """For each test, first load the default settings file, unless marked otherwise"""
+    if use_defaults:
+        helper.Settings.load_settings()
+
+
 def test_helper_altlink(monkeypatch):
     class Message:
         headers = {"content-type": "text/plain", "length": 300}
@@ -56,6 +63,7 @@ def test_helper_altlink(monkeypatch):
     assert newurl.startswith("http://ftp.jaist.ac.jp/")
 
 
+@pytest.mark.load_default_settings(False)
 def test_settings(tmp_path):
     helper.Settings.load_settings(os.path.join(os.path.dirname(__file__), "data", "settings.ini"))
     assert helper.Settings.concurrency == 3