go/types: nest all test data under the testdata directory

Having multiple subdirectories of go/types containing test data is
slightly problematic:
 - If ever we were to include a .go file in one of these directories,
   we'd inadvertently create a visible package.
 - It's difficult to add other content in testdata/, since TestTestdata
   scans the entire directory.

Move everything down a level, into testdata/{fixedbugs,examples,check},
and update tests accordingly.

Change-Id: Idd074c94b7b261d678934330539e41a48c2a9dc9
Reviewed-on: https://go-review.googlesource.com/c/go/+/314829


Trust: Robert Findley <rfindley@google.com>
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>