Can't set rlang_trace_top_env for expected errors

[lionel-]

This affects testthat snapshots in particular, see what happens to rlang backtrace tests with dev evaluate:

https://github.com/r-lib/rlang/compare/tmp/evaluate-snapshots?expand=1

I don't see a way of accommodating rlang_trace_top_env with the new evaluation setup. Possible solutions:

1. Revert the new evaluation scheme and give up on on.exit.

2. Add an option to trim rlang backtraces to evaluate(), following the approach discussed in Regression with last_error() in knitr #220:

   • Insert a dummy frame on the stack to serve as sentinel
   • Add a variant of rlang_trace_top_env on rlang that indicates the parent of the top env. This would be set to the dummy frame.

   If this new argument if off by default, this not only requires a release of rlang prior to releasing evaluate, but also releases of knitr and testthat once evaluate is on CRAN to use the new argument. But we can probably make it the default.