Add IDL union example

[jwillemsen]

* tests/union/test.idl:

[codecov]

Codecov Report

Merging #216 (770fd9f) into master (b3a2910) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #216   +/-   ##
=======================================
  Coverage   59.60%   59.60%           
=======================================
  Files         572      572           
  Lines       47515    47515           
  Branches     4187     4187           
=======================================
  Hits        28322    28322           
  Misses      16121    16121           
  Partials     3072     3072           

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