New package for `retry` and `tempDir`

[DavidVorick]

We use these two functions all over the place in our codebase. Is this something that we would want to split into a separate package instead of re-implementing in every project?

I was thinking we could have like a testhelp package.

[huetsch]

There are 2 redundant functions in the Sia codebase, retry() in api/server_helpers_test.go and Retry() in build/testing.go. Sometimes both of them are even called in the same function. build.Retry also very recently snuck into some into some non-test code as well (https://github.com/NebulousLabs/Sia/blob/master/modules/renter/contractor/journal.go#L115). If journal's use of it is correct, testhelp probably isn't a good package name for it.

[DavidVorick]

Maybe retry should just be it's own package then. I could see some non-testing reasons to use it.