Automatically clean up testhelper.Context

testhelper.Context() currently return a cancellation function as a second return value. Majority of the tests do not need to explicitly cancel the context but they simply defer its cancellation to clean up after the test. Given this, we can reduce the test verbosity and make testhelper.Context easier to compose by removing the unnecessary second return value. This adds a t.Cleanup function to automatically cancel the context at the end of the tests and omits the returned cancellation function.

Tests which simply defer cancel() have had the extra call removed. Some tests explicitly call the cancellation, and these tests have been modified to add context.WithCancel around the testhelper.Context call. There are a few loctions where testing.TB was passed down to test helpers that create the context.