test/stargazers_count.js: initial unit test.

Test the stargazers_count() function. This test covers the 3 main
cases we'll come across: user with repos, user without repos, and
unknown user.

Test file selection is kind of a kludge. It requires the test
functions be in a specific order to match the array of test
files and response codes. Future work might be to set the test
file name and response code in each test.