go/types, types2: move shared tests into internal/types/testdata

go/types, types2: move shared tests into internal/types/testdata

This CL moves the directories check, examples, fixedbugs, and spec
from inside go/types/testdata to internal/types/testdata. Except
for the directory adjustments to check_test.go files, this is a
pure file move.

With this CL, both type checkers now share identical tests in an
independent location.

Fixes #54511.

Change-Id: Ib335692d927e93867a158b338f105c2b87e74dbe
Reviewed-on: https://go-review.googlesource.com/c/go/+/427674
Reviewed-by: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>