[dev.typeparams] go/types: add tests from dev.go2go

Add tests from the dev.go2go branch, modified to eliminate support for
parenthesized type embedding and method type parameters. For the most
part these tests were made to pass via the fixes from preceding CLs in
this stack.

While integrating support for type parameters with the changes to
go/types in master, a decision was made to temporarily use an error code
of 0 for new error messages. Now that these messages are actually
emitted during checking of test packages, it is a test failure for them
to have an error code of 0. To satisfy the test, create a new temporary
error code '_Todo', which represents an error code that has not yet been
assigned. _Todo is added only where it was necessary to make tests pass:
many error codes were left as 0, meaning we don't have any tests that
produce them. This marker may help us produce more comprehensive tests
in the future.

Finally, each package checked by testDir was made into a subtest, for
the ease of running individual packages while debugging test failures.
This seemed worth keeping.

Change-Id: Iba421b797e9fb11af664a73902f67d6c4f30ecad
Reviewed-on: https://go-review.googlesource.com/c/go/+/283854


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