Freshen up the tests around crashing

Motivated by:

Dear maintainer,

Please see the problems shown on https://cran.r-project.org/web/checks/check_results_reprex.html.

Please correct before 2024-07-07 to safely retain your package on CRAN.

This refers to the fact that, at the time of writing, callr:::crash() does not crash on R devel, but instead throws an error. So reprex's tests around its behaviour when R crashes are failing on CRAN's checks on R-devel.

I'm updating to use a different way of crashing R (but @gaborcsardi says he'll probably update callr's R-crashing code) and also adding skip_on_cran(), as it seems a bit fraught to test crashing R there.

══ Failed tests ════════════════════════════════════════════════════════════════
    ── Failure ('test-reprex.R:67:3'): reprex() errors for an R crash, by default ──
    `reprex(input = code)` did not throw the expected error.
    ── Failure ('test-reprex.R:78:3'): reprex() copes with an R crash, when `std_out_err = TRUE` ──
    `out` does not match "segfault".
    Actual values:
    * ``` r
    * utils::getFromNamespace\("crash", "callr"\)\(\)
    * #> Error in get\("attach"\)\(structure\(list\(\), class = "UserDefinedDatabase"\)\): R_ExternalPtrAddr: argument of type VECSXP is not an external pointer
    * ```
    * 
    * <sup>Created on 2024-07-02 with \[reprex v2\.1\.0\]\(https://reprex\.tidyverse\.org\)</sup>
    * 
    * <details style="margin-bottom:10px;">
    * 
    * <summary>
    * 
    * Standard output and standard error
    * </summary>
    * 
    * 
    * ``` sh
    * -- nothing to show --
    * ```
    * 
    * </details>
    Backtrace:
        ▆
     1. └─testthat::expect_match(out, "segfault", all = FALSE) at test-reprex.R:78:3
     2.   └─testthat:::expect_match_(...)
    ── Failure ('test-reprex.R:79:3'): reprex() copes with an R crash, when `std_out_err = TRUE` ──
    `out` does not match "Traceback".
    Actual values:
    * ``` r
    * utils::getFromNamespace\("crash", "callr"\)\(\)
    * #> Error in get\("attach"\)\(structure\(list\(\), class = "UserDefinedDatabase"\)\): R_ExternalPtrAddr: argument of type VECSXP is not an external pointer
    * ```
    * 
    * <sup>Created on 2024-07-02 with \[reprex v2\.1\.0\]\(https://reprex\.tidyverse\.org\)</sup>
    * 
    * <details style="margin-bottom:10px;">
    * 
    * <summary>
    * 
    * Standard output and standard error
    * </summary>
    * 
    * 
    * ``` sh
    * -- nothing to show --
    * ```
    * 
    * </details>
    Backtrace:
        ▆
     1. └─testthat::expect_match(out, "Traceback", all = FALSE) at test-reprex.R:79:3
     2.   └─testthat:::expect_match_(...)

    [ FAIL 3 | WARN 0 | SKIP 65 | PASS 60 ]
    Error: Test failures
    Execution halted

tidyverse / reprex

Freshen up the tests around crashing #464