fix: require strategic interface in each livecheck strategy

[dduugg]

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…otherwise individual test runs will fail, e.g.

$ brew tests --only livecheck/strategy/gnome
Randomized with seed 14538
1 process for 1 spec, ~ 1 spec per process

An error occurred while loading ./test/livecheck/strategy/gnome_spec.rb.
Failure/Error: extend Strategic

NameError:
  uninitialized constant Homebrew::Livecheck::Strategy::Gnome::Strategic
# ./livecheck/strategy/gnome.rb:28:in `<class:Gnome>'
# ./livecheck/strategy/gnome.rb:27:in `<module:Strategy>'
# ./livecheck/strategy/gnome.rb:6:in `<module:Livecheck>'
# ./livecheck/strategy/gnome.rb:5:in `<module:Homebrew>'
# ./livecheck/strategy/gnome.rb:4:in `<top (required)>'
# ./test/livecheck/strategy/gnome_spec.rb:3:in `<top (required)>'