aes_gcm: Add counter test for max input/output length.

[briansmith]

For ring 0.17.12, both of these tests pass for default release mode,
release mode with overflow-checks=true, and debug mode, on 64-bit and
32-bit targets.

For prior releases, test_aes_gcm_counter_blocks_max_minus_one passes
in all three modes.

For prior releases, test_aes_gcm_counter_blocks_max passes in default
release mode:

$ cargo test --lib test_aes_gcm_counter_blocks --release
[snip]
test aead::aes::aes_gcm_tests::test_aes_gcm_counter_blocks_max ... ok
test aead::aes::aes_gcm_tests::test_aes_gcm_counter_blocks_max_minus_one ... ok

But, for 64-bit targets, when overflow checks are enabled (including debug mode),
test_aes_gcm_counter_blocks_max fails:

$ RUSTFLAGS="-C overflow-checks=true" cargo test --lib test_aes_gcm_counter_blocks
[snip]
thread 'aead::aes::aes_gcm_tests::test_aes_gcm_counter_blocks_max' panicked at src\aead\aes.rs:157:25:
attempt to add with overflow

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.59%. Comparing base (7b06be8) to head (d04d7b4).
Report is 4 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2455   +/-   ##
=======================================
  Coverage   96.59%   96.59%           
=======================================
  Files         180      180           
  Lines       21748    21780   +32     
  Branches      538      539    +1     
=======================================
+ Hits        21007    21039   +32     
  Misses        623      623           
  Partials      118      118           

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