[clang] [clang][NFC] Reduce one of JSON dump tests (PR #195026)

[Matheus Izvekov via cfe-commits]

https://github.com/mizvekov requested changes to this pull request.

The whole point of this json test was not just these flags.

We had low coverage of some template stuff in json test, and this was a drive by improvement.

For example, since this test was added, I have relied on it to test multiple other PRs.

https://github.com/llvm/llvm-project/pull/195026