test: use kani built-in --harness-timeout option

[roypat]

Set a per-harness timeout of 40 minutes. This should make it easier to debug scenarios where proofs end up hanging indefinitely, as we now no longer hit the pytest timeout, but instead a timeout built into kani (which should tell us what exact harness timed out), e.g.:

Thread 95:
CBMC failed
VERIFICATION:- FAILED
CBMC timed out. You may want to rerun your proof with a larger timeout or use stubbing to reduce the size of the code the verifier reasons about.
Manual Harness Summary:
Verification failed for - devices::virtio::iovec::verification::verify_write_to_iovec
Complete - 34 successfully verified harnesses, 1 failures, 35 total.

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---

• This functionality cannot be added in rust-vmm.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.90%. Comparing base (d5f3513) to head (c34953d).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5252      +/-   ##
==========================================
+ Coverage   82.84%   82.90%   +0.05%     
==========================================
  Files         250      250              
  Lines       26967    26967              
==========================================
+ Hits        22342    22356      +14     
+ Misses       4625     4611      -14     

Flag	Coverage Δ
5.10-c5n.metal	83.33% <ø> (+<0.01%)	⬆️
5.10-m5n.metal	83.33% <ø> (ø)
5.10-m6a.metal	82.55% <ø> (ø)
5.10-m6g.metal	79.16% <ø> (ø)
5.10-m6i.metal	83.33% <ø> (+<0.01%)	⬆️
5.10-m7a.metal-48xl	82.54% <ø> (?)
5.10-m7g.metal	79.16% <ø> (ø)
5.10-m7i.metal-24xl	83.30% <ø> (?)
5.10-m7i.metal-48xl	83.29% <ø> (?)
5.10-m8g.metal-24xl	79.16% <ø> (?)
5.10-m8g.metal-48xl	79.16% <ø> (?)
6.1-c5n.metal	83.37% <ø> (ø)
6.1-m5n.metal	83.38% <ø> (+<0.01%)	⬆️
6.1-m6a.metal	82.59% <ø> (ø)
6.1-m6g.metal	79.16% <ø> (ø)
6.1-m6i.metal	83.38% <ø> (+<0.01%)	⬆️
6.1-m7a.metal-48xl	82.59% <ø> (?)
6.1-m7g.metal	79.16% <ø> (-0.01%)	⬇️
6.1-m7i.metal-24xl	83.38% <ø> (?)
6.1-m7i.metal-48xl	83.39% <ø> (?)
6.1-m8g.metal-24xl	79.16% <ø> (?)
6.1-m8g.metal-48xl	79.16% <ø> (?)

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