Better test function for error checking normalize_entity

[gsheni]

• Added a more clear test for normalize_entity and make_time_index
  • We have an error check for make_time_index to make sure that the column is in copy_variables or additional_variables
  • I rewrote the test cases to more clearly check this, and it uses another entity (customers instead of sessions)

[codecov]

Codecov Report

Merging #590 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #590      +/-   ##
==========================================
+ Coverage   96.43%   96.44%   +<.01%     
==========================================
  Files         117      117              
  Lines        9521     9526       +5     
==========================================
+ Hits         9182     9187       +5     
  Misses        339      339

Impacted Files	Coverage Δ
featuretools/tests/entityset_tests/test_es.py	100% <100%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update cde74cf...3cd58d6. Read the comment docs.

[rwedge]

Since the error text mentions the make_time_index must be in additional_variables or copy_variables can we add a test where it's in copy_variables

[rwedge]

Looks good