abort botan tests when an assertion is not met

[lz101010]

See the following PRs relating to fuzz testing botan's TLS 1.3:

• https://github.com/randombit/botan/pull/3013
• https://github.com/randombit/botan/pull/2980

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[jonathanmetzman]

@randombit Do you approve?

[randombit]

@jonathanmetzman Sorry about delay, yes looks good to me

[lz101010]

Sorry about requesting approvals again, it seems that in the meantime there was at least one other PR which was merged and the workflow setup now considers my branch stale.