wait for uvicorn server to bind before running tests

Previously we had a simple `time.sleep(1)` call after `server.start()`
which was present to give the Mint's HTTP server time to spin up during
test runs. This meant that if the server took longer than 1s to start
on a dev's machine for any reason (even intermittently) then tests would
fail due to connection errors.

The fix is to use a simple repeated polling check which allows
the test runner to start only once the server is confirmed listening.

tests/conftest.py

@@ -27,7 +27,6 @@
 settings.cashu_dir = "./test_data/"
 settings.mint_host = "localhost"
 settings.mint_port = SERVER_PORT
-settings.mint_host = "0.0.0.0"
 settings.mint_listen_port = SERVER_PORT
 settings.mint_url = SERVER_ENDPOINT
 settings.tor = False
@@ -139,6 +138,16 @@ def mint():
 
     server = UvicornServer(config=config)
     server.start()
-    time.sleep(1)
+
+    # Wait until the server has bound to the localhost socket. Max out after 10s.
+    tries = 0
+    while tries < 100:
+        try:
+            httpx.get(settings.mint_url)
+        except:
+            tries += 1
+            time.sleep(0.1)
+
+
     yield server
     server.stop()