search_remote_tap spec: fix test offline

[mistydemeo]

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As pointed out by @JCount, the method being tested always returns [] immediately if HOMEBREW_NO_GITHUB_API is set even though we stub the JSON response. As a result, we get an expected [] to match ["homebrew/foo/some-formula"] failure when --online isn't passed. Unsetting HOMEBREW_NO_GITHUB_API for this one test should be safe.

[reitermarkus]

Thanks!