Windows 1809 unit tests are sometimes cut off

In https://gitlab.com/gitlab-org/gitlab-runner/-/jobs/2594811878#L1311 we can see that there's a line --- Starting part 1 of go unit tests of 'gitlab.com/gitlab-org/gitlab-runner/shells' package: after which a lot of tests are ran. However in another similar job this line is missing: https://gitlab.com/gitlab-org/gitlab-runner/-/jobs/2602863988#L1502. This caused us to not catch a failing test (#29125)