Use local copy of demo model for testing.

In an effort to keep all test fixtures in one place, a copy of the
tutorial model `capacitor.mph` from the `demos` folder was placed in
the `tests` folder, under the name `demo.mph`.

That file is used by very few tests: Only for loading files from disk
when testing the client, and when comparing tags created by MPh with
those generated by Comsol. All other tests create the (same) model
from scratch, via the `capacitor()` fixture in `models.py`.

tests/test_client.py

@@ -15,7 +15,7 @@
 ########################################
 client = None
 model  = None
-demo   = Path(__file__).resolve().parent.parent/'demos'/'capacitor.mph'
+demo   = Path(__file__).resolve().parent/'demo.mph'
 
 
 ########################################
@@ -63,7 +63,7 @@ def test_repr():
 
 def test_contains():
     assert model in client
-    assert 'capacitor' in client
+    assert 'demo' in client
     assert 'empty' in client
     assert 'non-existing' not in client
 
@@ -74,7 +74,7 @@ def test_iter():
 
 
 def test_truediv():
-    assert client/'capacitor' == model
+    assert client/'demo' == model
     with logging_disabled():
         with raises(ValueError):
             client/'non-existing'

tests/test_node.py

@@ -99,7 +99,7 @@ def test_tag():
     if not client.port:
         # Skip test in client-server mode where it's fairly slow.
         here = Path(__file__).resolve().parent
-        demo = client.load(here.parent/'demos'/'capacitor.mph')
+        demo = client.load(here/'demo.mph')
         demo.solve()
         root = Node(model, '')
         compare_tags(root, demo)