Move test definition behind a trait

[rydrman]

The first of a few refactorings to come. The goal here is to hide away the v0::TestSpec that was being used so that the new v1 package can use an alternate format for structuring and defining tests.

The other notable change is the logic for selecting which tests to run has become a function of the recipe itself, which will allow the v1 spec to exercise its richer when statements in this context.

Builds on #530
Part of #296

[codecov]

Codecov Report

Merging #560 (a260882) into main (0943b48) will decrease coverage by 0.01%.
The diff coverage is 71.62%.

@@            Coverage Diff             @@
##             main     #560      +/-   ##
==========================================
- Coverage   53.68%   53.67%   -0.01%     
==========================================
  Files         237      238       +1     
  Lines       19101    19119      +18     
==========================================
+ Hits        10254    10263       +9     
- Misses       8847     8856       +9     

Impacted Files	Coverage Δ
crates/spk/src/cli.rs	0.00% <ø> (ø)
crates/spk-schema/src/v0/spec.rs	84.41% <44.44%> (-2.58%)	⬇️
crates/spk-schema/src/recipe.rs	34.78% <66.66%> (ø)
crates/spk-schema/src/spec.rs	79.41% <70.00%> (-1.06%)	⬇️
crates/spk-schema/src/test.rs	72.00% <72.00%> (ø)
crates/spk-cli/cmd-test/src/cmd_test.rs	85.39% <100.00%> (+5.18%)	⬆️
crates/spk-cli/common/src/flags.rs	22.36% <100.00%> (-1.43%)	⬇️
crates/spk-schema/src/v0/test_spec.rs	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes

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[jrray]

Small nit!