Adding CHECKED_OUT_VERSION to .gitignore.

Adding CHECKED_OUT_VERSION in test/simdjs so that
the bots don't revert and redownload each time.

LOG=N
BUG=None
TEST=None
R=littledan@chromium.org,machenbach@chromium.org

Review URL: https://codereview.chromium.org/1242863003

Cr-Commit-Position: refs/heads/master@{#29826}

.gitignore

@@ -60,6 +60,7 @@ shell_g
 /test/promises-aplus/promises-tests
 /test/promises-aplus/promises-tests.tar.gz
 /test/promises-aplus/sinon
+/test/simdjs/CHECKED_OUT_*
 /test/simdjs/ecmascript_simd*
 /test/simdjs/data*
 /test/test262/data