test: Ensure that capellambse is installed

Raise an error if we detect that the built-in model extensions weren't
found in the entry points, as that would lead to a lot of follow-up
errors in the test suite.

tests/conftest.py

@@ -2,6 +2,7 @@
 # SPDX-License-Identifier: Apache-2.0
 """Global fixtures for pytest."""
 import collections.abc as cabc
+import importlib.metadata as imm
 import io
 import pathlib
 import sys
@@ -17,6 +18,15 @@
 
 capellambse.load_model_extensions()
 
+if not any(
+    i.module.startswith("capellambse.extensions.")
+    for i in imm.entry_points(group="capellambse.model_extensions")
+):
+    raise RuntimeError(
+        "Built-in model extensions are not loaded,"
+        " is capellambse installed properly?"
+    )
+
 
 @pytest.fixture(scope="session")
 def session_shared_model() -> cabc.Iterator[capellambse.MelodyModel]: