testing: use reflect.TypeAssert and reflect.TypeFor
To simplify the code.
Updates #62121
Updates #60088
Change-Id: I1cef27b19961721b89a965c708320072a052d9af
GitHub-Last-Rev:
ffd07a74da87b84b457246b2c94d4ee7540c8e8b
GitHub-Pull-Request: golang/go#75494
Reviewed-on: https://go-review.googlesource.com/c/go/+/704575
Reviewed-by: Alan Donovan <adonovan@google.com>
Auto-Submit: Alan Donovan <adonovan@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>