Fix resetting y_holdout index for time series

[eccabay]

Closes #4159

[codecov]

Codecov Report

Merging #4161 (865d6df) into main (2eee8cb) will increase coverage by 0.1%.
The diff coverage is 100.0%.

@@           Coverage Diff           @@
##            main   #4161     +/-   ##
=======================================
+ Coverage   99.7%   99.7%   +0.1%     
=======================================
  Files        349     349             
  Lines      37777   37797     +20     
=======================================
+ Hits       37660   37680     +20     
  Misses       117     117             

Impacted Files	Coverage Δ
evalml/pipelines/time_series_pipeline_base.py	100.0% <100.0%> (ø)
.../tests/pipeline_tests/test_time_series_pipeline.py	100.0% <100.0%> (ø)

[jeremyliweishih]

LGTM - thank you!

[chukarsten]

Thanks for tracking this down! Aside from the internal terminology/variable naming, this looks g2g.