More consistently use modules for unit tests

[djc]

(tests is the convention, as demonstrated by looking at the code produced by cargo init --lib.)

[codecov]

Codecov Report

Merging #1470 (588d6a2) into main (a1950e8) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1470      +/-   ##
==========================================
- Coverage   96.43%   96.43%   -0.01%     
==========================================
  Files          72       72              
  Lines       15163    15161       -2     
==========================================
- Hits        14622    14620       -2     
  Misses        541      541              

Files Changed	Coverage Δ
rustls/src/bs_debug.rs	100.00% <ø> (ø)
rustls/src/client/handy.rs	98.80% <ø> (ø)
rustls/src/crypto/ring/quic.rs	99.68% <ø> (ø)
rustls/src/dns_name.rs	100.00% <ø> (ø)
rustls/src/hash_hs.rs	100.00% <ø> (ø)
rustls/src/hkdf.rs	100.00% <ø> (ø)
rustls/src/key_log_file.rs	97.80% <ø> (ø)
rustls/src/limited_cache.rs	100.00% <ø> (ø)
rustls/src/msgs/mod.rs	100.00% <ø> (ø)
rustls/src/server/handy.rs	100.00% <ø> (ø)
... and 9 more

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