[druid] Updating Druid refresh metadata tests

[john-bodley]

This PR updates the Druid tests for refreshing of the metadata in the following ways:

• Removes the unnecessary enumerating of datasources as there's only one.
• Adds a check that the column type also is updated during the refresh. Previously we only checked the updated metrics.

to: @GabeLoins

[codecov-io]

Codecov Report

Merging #4887 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #4887   +/-   ##
=======================================
  Coverage   77.13%   77.13%           
=======================================
  Files          44       44           
  Lines        8542     8542           
=======================================
  Hits         6589     6589           
  Misses       1953     1953

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[mistercrunch]

LGTM