doc: add node-test-commit-custom-suites to docs
Add node-test-commit-custom-suites to the COLLABORATOR_GUIDE.
Refs: https://github.com/nodejs/node/pull/21909#issuecomment-406796234
Checklist
-
make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
documentation is changed or added -
commit message follows commit guidelines
/ping @lance @maclover7