Do not allow to provide absolute paths as filesystem will join it silently (fixes #211).

tests/nodes/io/test_io_base.py

@@ -0,0 +1,12 @@
+import pytest
+
+from bonobo.nodes.io.base import filesystem_path
+
+
+def test_filesystem_path_absolute():
+    with pytest.raises(ValueError):
+        filesystem_path("/this/is/absolute")
+
+
+def test_filesystem_path_relative():
+    assert filesystem_path("this/is/relative") == "this/is/relative"