add test fixtures for string and date methods

[cs01]

Summary

• Add test fixtures for string.substring(), string.includes(), string.charCodeAt(), string.substr()
• Add test fixture for Date.getHours(), Date.getMinutes(), Date.getSeconds(), Date.getDate()
• Fills coverage gaps for string manipulation and date instance methods

Test plan

• All 5 new fixtures pass locally with node compiler
• npm run verify:quick passes