[refactor] move tests to tests/, add pytest functions and main() entrypoints

- Moved src/tests/ → tests/
- Added test_* functions with assertions to script-style test files
- Added main() to each so IDEs offer it as a separate run target from pytest
- Fixed cupy_test.py: remove spurious x_gpu += x_cpu, fix duplicate xlabel

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/test_sub_image.py

@@ -0,0 +1,26 @@
+from image.sub_image import SubImageExtract
+from rbm.matrix import np
+
+
+def test_sub_image_rgb():
+	n, c, w, h = 2, 3, 8, 8
+	img = np.reshape(np.linspace(1, n*c*w*h, num=n*c*w*h, dtype=np.float64), (n, c, w, h))
+	sub = SubImageExtract(4, 4, 1, 1)
+	result = sub(img, 3)
+	assert result.shape[1] == 3
+	assert result.shape[2] == 4 and result.shape[3] == 4
+
+
+def test_sub_image_gray():
+	n, w, h = 2, 8, 8
+	img = np.reshape(np.linspace(1, n*w*h, num=n*w*h, dtype=np.float64), (n, w, h))
+	sub = SubImageExtract(4, 4, 1, 1)
+	result = sub(img, 1)
+	assert result.shape[1] == 1
+	assert result.shape[2] == 4 and result.shape[3] == 4
+
+
+if __name__ == "__main__":
+	test_sub_image_rgb()
+	test_sub_image_gray()
+	print("Test: [passed]")