On a few occasions I've had to read timeout stack traces for tests and
noticed that retry.Run runs the function in a goroutine. This makes
debuging a timeout more difficult because the gourinte of the retryable
function is disconnected from the stack of the actual test. It requires
searching through the entire stack trace to find the other goroutine.
By using panic instead of runtime.Goexit() we remove the need for a
separate goroutine.
Also a few other small improvements:
* add `R.Helper` so that an assertion function can be used with both
testing.T and retry.R.
* Pass t to `Retryer.NextOr`, and call `t.Helper` in a number of places
so that the line number reported by `t.Log` is the line in the test
where `retry.Run` was called, instead of some line in `retry.go` that
is not relevant to the failure.
* improve the implementation of `dedup` by removing the need to iterate
twice. Instad track the lines and skip any duplicate when writing to
the buffer.
