testing: use reflect.TypeAssert and reflect.TypeFor

[apocelipes]

To simplify the code.

Updates #62121
Updates #60088

[gopherbot]

This PR (HEAD: ffd07a7) has been imported to Gerrit for code review.

Please visit Gerrit at https://go-review.googlesource.com/c/go/+/704575.

Important tips:

• Don't comment on this PR. All discussion takes place in Gerrit.
• You need a Gmail or other Google account to log in to Gerrit.
• To change your code in response to feedback:
  • Push a new commit to the branch used by your GitHub PR.
  • A new "patch set" will then appear in Gerrit.
  • Respond to each comment by marking as Done in Gerrit if implemented as suggested. You can alternatively write a reply.
  • Critical: you must click the blue Reply button near the top to publish your Gerrit responses.
  • Multiple commits in the PR will be squashed by GerritBot.
• The title and description of the GitHub PR are used to construct the final commit message.
  • Edit these as needed via the GitHub web interface (not via Gerrit or git).
  • You should word wrap the PR description at ~76 characters unless you need longer lines (e.g., for tables or URLs).
• See the Sending a change via GitHub and Reviews sections of the Contribution Guide as well as the FAQ for details.

[gopherbot]

Message from Alan Donovan:

Patch Set 1: Auto-Submit+1 Code-Review+2 Commit-Queue+1

(1 comment)

---

Please don’t reply on this GitHub thread. Visit golang.org/cl/704575.
After addressing review feedback, remember to publish your drafts!

[gopherbot]

Message from Go LUCI:

Patch Set 1:

Dry run: CV is trying the patch.

Bot data: {"action":"start","triggered_at":"2025-09-17T13:59:41Z","revision":"1eea3eec04fb59881878b538d5bfbef308644c63"}

---

Please don’t reply on this GitHub thread. Visit golang.org/cl/704575.
After addressing review feedback, remember to publish your drafts!

[gopherbot]

Message from Alan Donovan:

Patch Set 1: -Commit-Queue

---

Please don’t reply on this GitHub thread. Visit golang.org/cl/704575.
After addressing review feedback, remember to publish your drafts!

[gopherbot]

Message from Go LUCI:

Patch Set 1:

This CL has passed the run

---

Please don’t reply on this GitHub thread. Visit golang.org/cl/704575.
After addressing review feedback, remember to publish your drafts!

[gopherbot]

Message from Go LUCI:

Patch Set 1: LUCI-TryBot-Result+1

---

Please don’t reply on this GitHub thread. Visit golang.org/cl/704575.
After addressing review feedback, remember to publish your drafts!

[gopherbot]

Message from Michael Knyszek:

Patch Set 1: Code-Review+1

---

Please don’t reply on this GitHub thread. Visit golang.org/cl/704575.
After addressing review feedback, remember to publish your drafts!

[gopherbot]

This PR is being closed because golang.org/cl/704575 has been merged.