`absl::StrCat` always returns a `std::string`. Specifying it using a trailing return type does not help.
Tests are documentation, and they should be consistent with [the google style guide](https://google.github.io/styleguide/cppguide.html#trailing_return).

For example, this is different than `-> absl::string_view` and returning string literals, where it is actually a load-bearing change.

PiperOrigin-RevId: 636705683
Change-Id: I0d84a562a59bc0c16be01dd2ae5538adb401432e