Update 100-white-box-testing.md (#1801)

* Update 100-white-box-testing.md

Add description for white-box testing
Provide link to ISTQB glossary entry
Provide link to ISTQB syllabus with page to specific subject

* Update content/roadmaps/113-qa/content/100-qa-basics/102-testing-approaches/100-white-box-testing.md

Co-authored-by: Kamran Ahmed <kamranahmed.se@gmail.com>

content/roadmaps/113-qa/content/100-qa-basics/102-testing-approaches/100-white-box-testing.md

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-# White box testing
+# White Box Testing
+
+White Box Testing is a technique in which software’s internal structure, design, and coding are tested to verify input-output flow and improve design, usability, and security. In white box testing, code is visible to testers, so it is also called Clear box testing, Open box testing, Transparent box testing, Code-based testing, and Glass box testing.
+
+<ResourceGroupTitle>Free Content</ResourceGroupTitle>
+<BadgeLink colorScheme='yellow' badgeText='Read' href='https://www.guru99.com/white-box-testing.html'>White Box Testing – What is, Techniques, Example & Types</BadgeLink>