Fix TextIOWrapper assertion typo and simplify test

- Fix assertion to check skip_bytes instead of skip_back
- Use simpler test data b'line1\r' instead of b'line1=1\r'
- Remove unnecessary multiple CR test case
- Clean up workaround code

The assertion was checking wrong variable (skip_back vs skip_bytes).
skip_back is search step size, skip_bytes is buffer offset needing validation.

Author: mohsinm-dev