test,doc: do not indicate that non-functions "return" values

Update documentation for test `common` module so that properties that
are not functions do not specify a return type and functions that do
return values specify the type.

PR-URL: https://github.com/nodejs/node/pull/17267


Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>