Use exit code in addition of print for "test"

[kerneis]

The documentation mentions that "oauth2l test" returns 0 or 1, but the
current code prints the value instead of setting an exit code. This is
not useful for scripting.

This commit still keeps the printing part for backwards-compatibility
reasons.

[kerneis]

The Python version used to have no output, which is what the document mentions as well:
https://github.com/google/oauth2l/blob/master/python/oauth2l/oauth2l_test.py#L305

However, the switch to the go version in 7c859d8 happened more than a year ago. Although nobody opened a bug since then, Hyrum's law (https://www.hyrumslaw.com/) probably means that someone somewhere relies on it. This change restores the "exit code" behavior while keeping the print statement to ensure maximal backwards compatibility. Forked from discussion in #84.

[andyrzhao]

Looks good. Thanks for putting in this fix!