Add build targets for Opus tests.

Add build targets for Opus tests.

test_opus_api, test_opus_encode, test_opus_decode, and test_opus_padding
can be run directly, with no command-line options, and they print out
"OK" during the test and an overall status message such as
"All API tests passed." or "Tests completed successfully." at the end.

The celt component has several unit tests. I didn't add build targets
for them because some of them don't print any output, so you'd need a
test harness script to examine their exit status.

R=henrika@chromium.org,sergeyu@chromium.org
BUG=452984

Review URL: https://codereview.chromium.org/890473002

Cr-Commit-Position: refs/heads/master@{#313964}