cmd/compile/internal/types2: review of api_test.go

The changes between (equivalent, and reviewed) go/types/api_test.go
and api_test.go can be seen by comparing patchset 1 and 2. The actual
changes are removing the "// UNREVIEWED" marker, the addition of the
TestConvertibleTo and TestAssignableTo tests, and adjustments to test
prefixes (genericPkg, brokenPkg to be in line with go/types).

There are several differences to go/types/api_test.go:
- use of syntax rather than go/ast package
- use of the parseSrc helper function
- TestTypesInfo test entries reflect different handling of untyped nil
- TestInferredInfo is (for go1.17) in another file controlled by a build
  constraint in go/types
- TestSelection test is currently skipped (types2 position information
  is not accurate enough)
- TestScopeLookupParent doesn't have access to a scanner and instead
  relies on syntax.CommentsDo.
- Broken packages are assumed to contain generic code for the tests.

Change-Id: Ic14e6fb9d6bef5416df39e465b5994de76f84097
Reviewed-on: https://go-review.googlesource.com/c/go/+/304131


Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>