add unit test for metric 'pull_requests_merge_contributor_new'
add unit test for metric 'pull_requests_merge_contributor_new'

Signed-off-by: bing0n3 <dc76e9f0c0006e8f919e0c515c66dbba3982f785@bing0ne.com>

OSSHealth/ghdata,OSSHealth/ghdata,OSSHealth/ghdata