Update TESTING.md 'Building with extra plugins' documentation (#15893) * Update TESTING.md 'Building with extra plugins' documentation Signed-off-by: Finn Carroll <carrofin@amazon.com> * Typo Signed-off-by: Finn Carroll <carrofin@amazon.com> * Fix TOC Signed-off-by: Finn Carroll <carrofin@amazon.com> --------- Signed-off-by: Finn Carroll <carrofin@amazon.com>