test: more tests for generic methods

These tests verify that generic method type arguments are ordered
correctly for a variety of expression forms and dictionary kinds.

Change-Id: I067206d1e97d5350d6fcc3983f5a6ef24a782214
Reviewed-on: https://go-review.googlesource.com/c/go/+/764200
Reviewed-by: Robert Griesemer <gri@google.com>
Auto-Submit: Mark Freeman <markfreeman@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>