Unable to specify qa data attribute for GlDropdown's split(chevron) button

We can use split option to add a dedicated dropdown toggle button to GlDropdown (instead of having a chevron icon inside one button). Example: https://gitlab-org.gitlab.io/gitlab-ui/?path=/story/base-button--dropdown-split-button

We cannot currently pass down any css classes or data attributes to the dropdown toggle button and we couldn't keep all the QA selectors that were in use when migrating Bootstrap dropdowns to GlDropdown:

gitlab-org/gitlab!50933 (comment 481095446)

With the previous code, we attached a qa selector note_dropdown to the chevron button. It doesn't look like we can do this with GlDropdown since the chevron button is part of its implementation and we currently can't pass a qa selector to GlDropdown to attach to the chevron button. If there's no way to get this working with QA's help, we might have to figure out a separate solution like allowing GlDropdown to take in a qa selector for the chevron button.