Fix/480 1611 (#30)

* fix(Eventually): fixed go routine leak issues

Reference (original issue report): stretchr#1611

This fix involves a complete rewrite of assertions that run background
checks, with more idiomatic go routine codes and use of context.Context.

Affected assertions:

* Eventually
* Never
* EventuallyWithT

We no longer run go routines that are not waited for.
Timeouts and early exits are handled via context.Context.

CollecT.FailNow() now also cancels the context of the caller instead.

Significant changes:
* callers should not expect Eventually and others to
  complete in the case of a blocking condition function.
  In particular, the condition function should never wait for an event that is
  triggered after Eventually. Complex or long-running condition function
  may use t.Context() to exit early if the test is failed.
* callers may safely assume that only one go routine is executing the
  condition function (no race on writes)

* test: asserted stretch/testify#480 is fixed

Signed-off-by: Frederic BIDON <fredbi@yahoo.com>

* chore: regenerated packages and docs

Signed-off-by: Frederic BIDON <fredbi@yahoo.com>

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Signed-off-by: Frederic BIDON <fredbi@yahoo.com>

Author: fredbi