Rename CodexTest.cpp to CodecTest.cpp

CodexTest confuses me almost every time I try to open it. Renaming it
to what my brain thinks it should be called.
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Review URL: https://codereview.chromium.org/1897973004

diff --git a/tests/CodexTest.cpp b/tests/CodecTest.cpp
similarity index 100%
rename from tests/CodexTest.cpp
rename to tests/CodecTest.cpp