Include context in spec time tracking

What does this MR do?

Track test time spend in before/after in knapsack so that knapsack execution time per file is closer to the actual rspec duration.

In the comparison below, we can see that knapsack global execution time is closer to the total rspec duration.

Before: https://gitlab.com/gitlab-org/gitlab/-/jobs/1165636482#L9069

  "spec/models/user_spec.rb": 86.19563937187195,
  "spec/services/ci/register_job_service_spec.rb": 46.63532996177673,
  "spec/services/ci/create_downstream_pipeline_service_spec.rb": 34.43165564537048,
  "spec/policies/issue_policy_spec.rb": 26.279865503311157,
...
Knapsack global time execution for tests: 10m 24s
Finished in 14 minutes 12 seconds (files took 50.27 seconds to load)

After: https://gitlab.com/gitlab-org/gitlab/-/jobs/1165733781#L9067

  "spec/models/user_spec.rb": 119.9533200263977,
  "spec/services/ci/register_job_service_spec.rb": 59.751078605651855,
  "spec/services/ci/create_downstream_pipeline_service_spec.rb": 44.74184823036194,
  "spec/policies/issue_policy_spec.rb": 32.7114417552948,
...
Knapsack global time execution for tests: 17m 34s
Finished in 17 minutes 37 seconds (files took 1 minute 3.05 seconds to load)

Rleated to #327087 (closed)

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