testing: add T.Context() accessor

[bradfitz]

Can we add a func (*T) Context() context.Context accessor in the testing package?

And then make each test run with a context which is canceled when it finishes.

Then goroutines created by tests can select on the test's completion (pass or fail).

Thoughts?

[robpike]

Requirement: This CL should not add any dependencies to the testing package (other than context of course),

[bradfitz]

I'll prototype an implementation for review.

[broady]

Transitive imports from context are a subset of those from testing.

[dsnet]

Something to think about when implementing this. Currently, it is a bug when people call T.FatalX or T.SkipX inside a goroutine other than the main test function. The current behavior is subtly wrong, but we could consider having them cancel the context as a possible alternative to #15758

[rsc]

SGTM. Seems orthogonal to #15758. @dsnet, remember that cancelling a context is a polite request only. It doesn't force things to be torn down.

[gopherbot]

CL https://golang.org/cl/31724 mentions this issue.

[gopherbot]

CL https://golang.org/cl/32648 mentions this issue.