Support binary roundtripping of RZX files in __test

[kosarev]

Currently zx __test file.rzx only runs the RZX playback. The .z80 test also verifies binary roundtripping (match(image, file.encode())).

To support the same for RZX:

1. Add RZXFile.encode() that calls make_rzx(self.chunks).
2. Add re-compression support in make_rzx() for snapshot chunks that were originally compressed (currently we decompress on load and re-encode uncompressed, so byte-exact comparison fails for compressed files).
3. Update test_file() to call match(image, file.encode()) for RZXFile the same way it does for MachineSnapshot.