Add skips for tests requiring 'seasonal'

[MichaelChirico]

Now, these tests fail opaquely:

══ Failed tests ════════════════════════════════════════════════════════════════
── Failure (test-features.R:20:3): unit root features ──────────────────────────
ft$pp_pvalue not equal to 0.1.
target is NULL, current is numeric
── Failure (test-features.R:67:3): model based features ────────────────────────
as.list(ft) not equivalent to list(...).
Length mismatch: comparison on first 7 components
Component 2: Mean relative difference: 0.01292009
Component 3: Mean relative difference: 11.76622
Component 4: Mean relative difference: 0.9995659
Component 5: Mean relative difference: 3.041742
Component 6: Mean relative difference: 55.83438
Component 7: Mean relative difference: 0.2125561