tests/heapalloc_bytesio: Test for BytesIO with preallocates space.

tests/micropython/heapalloc_bytesio.py

+import uio
+import micropython
+
+data = b"1234" * 16
+buf = uio.BytesIO(64)
+
+micropython.heap_lock()
+
+buf.write(data)
+
+micropython.heap_unlock()
+
+print(buf.getvalue())

tests/micropython/heapalloc_bytesio.py.exp

+b'1234123412341234123412341234123412341234123412341234123412341234'