Add rationale for ensure_cratedb_layer

Author: amotl

src/crate/client/tests.py

@@ -117,6 +117,17 @@ def setUpMocked(test):
 
 
 def ensure_cratedb_layer():
+    """
+    In order to skip individual tests by manually disabling them within
+    `def test_suite()`, it is crucial make the test layer not run on each
+    and every occasion. So, things like this will be possible::
+
+        ./bin/test -vvvv --ignore_dir=testing
+
+    TODO: Through a subsequent patch, the possibility to individually
+          unselect specific tests might be added to `def test_suite()`
+          on behalf of environment variables.
+    """
     global crate_layer
 
     if crate_layer is None: