feat: run unit tests with --release flag

[ShadowCurse]

Summary of the PR

Rust does optimizations which can cause havoc in --release mode, so add another set of unit tests
with all optimizations enabled.

See: rust-vmm/kvm#298

Requirements

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• All commits in this PR have Signed-Off-By trailers (with
  git commit -s), and the commit message has max 60 characters for the
  summary and max 75 characters for each description line.
• All added/changed functionality has a corresponding unit/integration
  test.
• All added/changed public-facing functionality has entries in the "Upcoming
  Release" section of CHANGELOG.md (if no such section exists, please create one).
• Any newly added unsafe code is properly documented.

[stefano-garzarella]

The changes LGTM, but I have a couple of questions:

1. should we run unit tests for both debug and release? (not sure if for the same reason, running tests only with optimizations, can hide some issues)
2. should we do the same for coverage? (in integration_tests/test_coverage.py we call cargo llvm-cov test without --release)

[ShadowCurse]

Added unit tests with debug build back.
Regarding coverage, I would assume this is not impacted by the optimization level, so no reason to add it there.

[stefano-garzarella]

Added unit tests with debug build back.

Can you also update commit and PR title/description?

Regarding coverage, I would assume this is not impacted by the optimization level, so no reason to add it there.

I see, definitely not to be fixed in this PR, but your changes made me wonder if it makes sense to calculate coverage by considering, for example, code that is not used in release (maybe we don't even have it, but it occurred to me). I'll open an issue to discusse that