split node tests into standalone files

tests/node_tests/test_mask_invert.py

@@ -0,0 +1,17 @@
+import numpy as np
+
+
+def test_mask_invert():
+    from backend.nodes.mask_invert import MaskInvert
+    node = MaskInvert()
+
+    mask = np.zeros((64, 64), dtype=np.uint8)
+    mask[10:20, 10:20] = 255
+
+    inverted, = node.process(mask)
+    assert inverted.dtype == np.uint8
+    assert np.all(inverted[10:20, 10:20] == 0)
+    assert np.all(inverted[0:10, 0:10] == 255)
+
+    double, = node.process(inverted)
+    assert np.array_equal(double, mask)