types2: add *Config to typecheck functions for tests, factor more code

types2: add *Config to typecheck functions for tests, factor more code

Some tests in go/types can still not use the typecheck helper functions
because they need a specific fileset for position information.

(We could use a single global file set for all tests to make this work.)

Change-Id: I73552b08a00f08d809c319d3d2328acee9532619
Reviewed-on: https://go-review.googlesource.com/c/go/+/461694
Auto-Submit: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
Run-TryBot: Robert Griesemer <gri@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>