Update test_all_estimators to account for Prophet in 3.9

[ParthivNaresh]

Fixes #2893
Adds 1 to the estimator count when running tests for python 3.9

[codecov]

Codecov Report

Merging #2892 (7eda352) into main (717075f) will not change coverage.
The diff coverage is 100.0%.

@@          Coverage Diff          @@
##            main   #2892   +/-   ##
=====================================
  Coverage   99.7%   99.7%           
=====================================
  Files        302     302           
  Lines      28312   28312           
=====================================
  Hits       28216   28216           
  Misses        96      96           

Impacted Files	Coverage Δ
evalml/tests/pipeline_tests/test_pipelines.py	99.8% <100.0%> (ø)

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[bchen1116]

LGTM!

[angela97lin]

Thank you @ParthivNaresh! 🙏