Fixes:
- `start_value` and `length_value` should be casted to integers
- proper handling of utf-8 characters
- do not need to cast blob to string, as substr in blobs refers to byte
indexes and not char-indexes
Closes#3465
When adding tests for extensions, please follow these guidelines:
Tests that verify the internal logic or behavior of a particular extension should go into cli_tests/extensions.py.
Tests that verify how extensions interact with the database engine, such as virtual table handling, should be written
in TCL (see vtab.test as an example).
To check which extensions are available in TCL, or to add a new one, refer to the tester.tcl file and look at the extension_map.
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