tests: lib: json: add escape handling tests

Add extensive test coverage for JSON escape sequence handling to
ensure robustness and prevent regression in string encoding/decoding
operations.

The new test suite includes:

1. **Escape Sequence Stability**: Tests multiple encode/decode cycles
   to prevent backslash duplication bugs where sequences like \n, \t
   could be duplicated with each cycle.

2. **Individual Escape Sequences**: Comprehensive testing of each JSON
   escape type:
   - \n (newline), \t (tab), \r (carriage return), \f (form feed)
   - \b (backspace), \\ (backslash), \" (quote), \/ (slash)

3. **Quote Handling**: Special focus on quote escaping/unescaping to
   ensure proper JSON syntax and data integrity.

4. **Mixed Escape Sequences**: Complex scenarios with multiple
   different escape sequences in single strings to test edge cases.

5. **Multiple Cycle Stability**: Repeated encode/decode operations to
   simulate real-world usage patterns and detect gradual corruption.

6. **Backslash Duplication Detection**: Specific tests targeting the
   common bug where backslashes accumulate during multiple processing
   cycles.

7. **Round-trip Validation**: Complete encode → decode → encode cycles
   to verify data preservation.

8. **Edge Cases**: Testing boundary conditions like strings containing
   only quotes or complex escape combinations.

These tests use a new `escape_test_data` structure with both string
value and string buffer fields to verify proper handling of both
JSON_TOK_STRING and JSON_TOK_STRING_BUF descriptor types.

The test suite ensures that:
- Escape sequences are properly encoded in JSON output
- Escape sequences are correctly unescaped during parsing
- No data corruption occurs during multiple processing cycles
- Both string pointer and string buffer fields behave correctly
- Complex escape scenarios are handled robustly

Signed-off-by: BUDKE Gerson Fernando <gerson.budke@leica-geosystems.com>

Author: nandojve