Add tests for placeholder widget.

tests/test_placeholder.py

@@ -0,0 +1,15 @@
+import pytest
+
+from textual.widgets import Placeholder
+from textual.widgets._placeholder import InvalidPlaceholderVariant
+
+
+def test_invalid_placeholder_variant():
+    with pytest.raises(InvalidPlaceholderVariant):
+        Placeholder("this is clearly not a valid variant!")
+
+
+def test_invalid_reactive_variant_change():
+    p = Placeholder()
+    with pytest.raises(InvalidPlaceholderVariant):
+        p.variant = "this is clearly not a valid variant!"