Use non-zero value when initializing test buffers

[caio]

Hi!

Picking up from PRs #564 and #600:

• The constant is in the test module now as UNINITIALIZED_U8 with
  value 0xDE
• Left the lone zeroed usize buffer untouched

I tried to find every candidate in tests/ and found a few tests from
inside src/ too.

And I've opted to leave the buffers from max_input_tests! in
src/digest.rs untouched as they are about the input length, but let me
know if you prefer otherwise.

Aside: the README recommends running cargo test --no-default-features
prior to a PR but it fails to compile for me (Windows, WSL2 with Ubuntu
20.04) because the test_file! macro is gated by the alloc feature.
Running cargo test --no-default-features --features alloc works fine.

[caio]

Force-pushed an update to the branch due to a failed rustfmt check. That one should pass now :)

[codecov]

Codecov Report

Merging #1157 (dd831cb) into main (1c36465) will decrease coverage by 1.77%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1157      +/-   ##
==========================================
- Coverage   93.69%   91.92%   -1.78%     
==========================================
  Files         115      106       -9     
  Lines       17462    14248    -3214     
==========================================
- Hits        16361    13097    -3264     
- Misses       1101     1151      +50     

Impacted Files	Coverage Δ
src/test.rs	89.45% <ø> (+0.04%)	⬆️
tests/digest_tests.rs	95.19% <ø> (ø)
src/aead/chacha.rs	99.23% <100.00%> (-0.23%)	⬇️
src/ec/suite_b/ecdh.rs	100.00% <100.00%> (ø)
src/rsa/padding.rs	98.17% <100.00%> (+0.02%)	⬆️
src/rsa/signing.rs	97.10% <100.00%> (ø)
tests/aead_tests.rs	97.14% <100.00%> (-0.91%)	⬇️
tests/hkdf_tests.rs	95.08% <100.00%> (ø)
tests/hmac_tests.rs	94.91% <100.00%> (-0.25%)	⬇️
tests/pbkdf2_tests.rs	87.50% <100.00%> (ø)
... and 55 more

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