test framework: add app_fails and slurp_file to OpenSSL::Test

Several test recipes duplicate a helper that runs an openssl
application expecting a failure exit status and checks that the
stderr output matches a regular expression.  Add a shared app_fails
function so new recipes do not need to repeat it, together with a
slurp_file function for reading back a captured output file.

Assisted-by: Claude:claude-fable-5
Reviewed-by: Dmitry Belyavskiy <beldmit@gmail.com>
Reviewed-by: Tomas Mraz <tomas@openssl.foundation>
Merge-date: Mon Sep  7 14:15:49 2026
Merged-from: https://github.com/openssl/openssl/pull/32592