Install splitic from its canonical module path

What does this MR do?

Updates the module path for splitic from gitlab.com/ajwalker/splitic@latest to gitlab.com/gitlab-org/ci-cd/runner-tools/splitic@latest.

Why was this MR needed?

splitic is used to help parallelise our integration test CI jobs.

The old path stopped receiving updates when the module was relocated, so the Windows test jobs have been pinned to an old build. This update switches to the maintained version.

A couple of fixes and improvements (gitlab-org/ci-cd/runner-tools/splitic!2 (merged), gitlab-org/ci-cd/runner-tools/splitic!3 (merged)) have recently been merged into the tool's canonical repository. This change is necessary to install from the correct repository going forward and avail of any new improvements.

What's the best way to test this MR?

Inspect a Windows test job in the pipeline (e.g. windows 1809 unit tests or windows 1809 integration tests) and verify splitic is installed from the updated module path and the job completes successfully.

What are the relevant issue numbers?

N/A