doc: run tests before landing changes

The Collaborator Guide suggests running tests after pushing changes to
the main repository. Tests should be run before then so move the
instructions earlier.

Instructions are also simplified and re-written to not exclude Windows.

PR-URL: https://github.com/nodejs/node/pull/12416


Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Alexey Orlenko <eaglexrlnk@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>