[llvm] [github] Enable assertions on test workflow (PR #74849)

[Natalie Chouinard via llvm-commits]

sudonatalie wrote:

@tstellar A couple SPIR-V backend tests are XFAILed due to hitting assertions, so they're unexpectedly passing on the bot. We could change this for just SPIR-V tests but I'm presuming this is the preferred default for most others too?

https://github.com/llvm/llvm-project/pull/74849