Move and restructure the tests for the Contains impl

[rye]

Part of #38.

[codecov]

Codecov Report

Merging #54 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master      #54   +/-   ##
=======================================
  Coverage   97.78%   97.78%           
=======================================
  Files          10       10           
  Lines         451      451           
=======================================
  Hits          441      441           
  Misses         10       10

Impacted Files	Coverage Δ
src/tests/netaddr.rs	100% <ø> (ø)	⬆️
src/traits/contains.rs	100% <100%> (ø)

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