Software Development Life Cycle involves the complete Verification and Validation of a Process or a Project.
Whereas Software Testing Life Cycle involves only Validation. Analysis, Design, Software requirement specifications, Development Process (Coding and Application development), Testing Process (Preparation Of Test Plan, Preparation of Test cases, Testing, Bug reporting, Test Logs & Test Reports), Release and Maintenance.
Whereas Software Testing Life Cycle involves Preparation of Test Plan, Preparation of Test cases, Test execution, Bug reporting & Tracking, Regression Testing and Test Closure.
STLC is a Part of SDLC.
Software Development Life Cycle involves business requirement specifications.