Taskscaler (autoscaler) executor unit tests

Per https://gitlab.com/gitlab-com/runner-group/team-tasks/-/issues/180#tasks

The new Taskscaler-based executor needs unit tests.

• add Taskscaler interface and mock -- gitlab-org/fleeting/taskscaler!3 (merged)
• unit tests for provider.go -- !3633 (merged)
• unit tests for executor.go
• unit tests for acquisition.go (in dev @dbickford)

(created from https://gitlab.com/gitlab-com/runner-group/team-tasks/-/issues/181)