go/types, types2: consolidate helper functions for tests (cleanup)

go/types, types2: consolidate helper functions for tests (cleanup)

Instead of having various inconsistent helper functions, rely on
4 helper functions with consistent naming and parameters:

- parse and mustParse
- typecheck and mustTypecheck

Panic rather than call t.Fatal in the mustX functions to simplify
their use.

Use the new functions in tests consistently.

Change-Id: Ib19dc5cc470b51512c23c09df32c379dc3eb8f4b
Reviewed-on: https://go-review.googlesource.com/c/go/+/443757
Reviewed-by: Robert Griesemer <gri@google.com>
Run-TryBot: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>