theora content page : fix links

On this page: https://theora.org/content/

• replace http://v2v.cc/~j/theora_testsuite/ with https://media.xiph.org/video/tests/theora/, maybe also add https://wiki.xiph.org/TheoraTestsuite
• replace all occurrences of http://wiki.xiph.org with https://wiki.xiph.org