test: Design a test to load a backend without Qibojit

Since qibojit is always installed in the CI, we have to temporarily
remove it manually, and restore after the test.

tests/test_backends.py

@@ -1,11 +1,16 @@
+import importlib
 import platform
+import shutil
 import sys
+from pathlib import Path
+from tempfile import TemporaryDirectory
 
 import numpy as np
 import pytest
+import qibojit
 
 from qibo import construct_backend, gates, list_available_backends, set_backend
-from qibo.backends import MetaBackend
+from qibo.backends import GlobalBackend, MetaBackend, get_backend
 
 ####################### Test `matrix` #######################
 GATES = [
@@ -112,6 +117,28 @@ def test_construct_backend(backend):
     )
 
 
+@pytest.fixture
+def uninstall_qibojit():
+    p = Path(qibojit.__file__).parent
+    modules = list(sys.modules.keys())
+    for mod in modules:
+        if mod.startswith("qibojit"):
+            del sys.modules[mod]
+    with TemporaryDirectory() as tdir:
+        shutil.move(p, tdir)
+        yield
+        shutil.move(Path(tdir) / p.name, p)
+    importlib.import_module("qibojit")
+
+
+def test_default_backend(uninstall_qibojit):
+    """Reproducing https://github.com/qiboteam/qibo/issues/1424."""
+    # reset global backend
+    GlobalBackend._instance = None
+    # attempt loading
+    default_backend = get_backend()
+
+
 def test_list_available_backends():
     tensorflow = False if platform.system() == "Windows" else True
     qulacs = (