Fix flaky unit test - remove 'Markup' object in data

[mistercrunch]

example of the failed test
https://travis-ci.org/apache/incubator-superset/jobs/397891630

@williaster

[williaster]

@mistercrunch thanks for looking into this!

for the frontend modifications, are existing slices guaranteed to not have markup?

[graceguo-supercat]

LGTM

[graceguo-supercat]

I like this code change, simplify modified attribute. But I am not sure this will resolve unit test issue.

[williaster]

I agree, good change but I think the real problem is that tests modify the db, and tests all share the same db.

cc @john-bodley

[mistercrunch]

Went with @graceguo-supercat 's solution as far as fixing the tests go, but I feel like this cleans the regexes and we should get it through.

[codecov-io]

Codecov Report

Merging #5313 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #5313      +/-   ##
==========================================
+ Coverage   61.32%   61.33%   +<.01%     
==========================================
  Files         369      369              
  Lines       23488    23483       -5     
  Branches     2717     2713       -4     
==========================================
- Hits        14405    14404       -1     
+ Misses       9071     9067       -4     
  Partials       12       12

Impacted Files	Coverage Δ
...rset/assets/src/dashboard/actions/sliceEntities.js	32.25% <ø> (+1.95%)	⬆️
...t/assets/src/dashboard/reducers/getInitialState.js	0% <ø> (ø)	⬆️
...c/dashboard/components/dnd/AddSliceDragPreview.jsx	61.9% <ø> (+2.81%)	⬆️
superset/models/helpers.py	87.7% <100%> (-0.07%)	⬇️

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