test: each app keeps its tests in a tests package

131 test modules sat next to the code they test; instances alone had
51 of them beside views.py and utils.py. They move into a
tests/ package per app, and the JavaScript checks into
webvirtcloud/tests/js. Each non-empty tests.py becomes tests/test_views.py;
the empty generated stubs are removed.

Imports of the app's own modules become absolute, paths built from
__file__ go one level up, the live helper is instances.tests.livetest,
and coverage omits */tests/*. The suite discovers the same 960 tests as
before.

webvirtcloud/tests/test_instance_tabs_js.py

@@ -0,0 +1,23 @@
+"""The VM page opens the tab a redirect points to (static/js/instance_tabs.js).
+Its behaviour is checked with Node."""
+
+import shutil
+import subprocess
+import unittest
+from pathlib import Path
+
+from django.conf import settings
+
+BASE = Path(settings.BASE_DIR)
+
+
+class InstanceTabsTestCase(unittest.TestCase):
+    @unittest.skipUnless(shutil.which("node"), "node is not installed")
+    def test_instance_tabs_js(self):
+        result = subprocess.run(
+            ["node", str(BASE / "webvirtcloud/tests/js/instance_tabs.test.js"), str(BASE / "static/js/instance_tabs.js")],
+            capture_output=True,
+            text=True,
+            timeout=60,
+        )
+        self.assertEqual(result.returncode, 0, result.stderr)