For developers,
testing is a skill. This is typically white box testing that gets done concurrently or just after coding.
For a test team or QA (quality assurance) team member, there are certifications available:
From the QAI (Quality Assurance Institute) Global Institute:
CAST/Certified Associate in Software TestingCASQ/Certified Assoicate in Software QualityCSTE/Certified Software TesterCSQA/Certified Software Quality Analyst
From the ISTQB (International Software Testing Qualifications Board):
CTFL/Certified Tester Foundation LevelCTAL/Certified Tester Advanced Level