Set random_state arg in Data Splitter

[bchen1116]

fix #1471

By passing in an int rather than np.random.RandomState, we can ensure the resulting datasplits are equivalent so that the AutoMLSearch algorithm can accurately compare model performances.

[codecov]

Codecov Report

Merging #1579 (8da6a0d) into main (3eb27b8) will increase coverage by 0.1%.
The diff coverage is 100.0%.

@@            Coverage Diff            @@
##             main    #1579     +/-   ##
=========================================
+ Coverage   100.0%   100.0%   +0.1%     
=========================================
  Files         240      240             
  Lines       18030    18055     +25     
=========================================
+ Hits        18022    18047     +25     
  Misses          8        8             

Impacted Files	Coverage Δ
evalml/automl/automl_search.py	99.7% <100.0%> (+0.1%)	⬆️
evalml/tests/automl_tests/test_automl.py	100.0% <100.0%> (ø)

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[jeremyliweishih]

wow!