test(common): support isConformant for portals

[levithomason]

These commits were cherry picked from Datetime #1240. We need to be able to test conformance for components that render portals.

• Add a rendersPortal isConformant option.
• Skip as prop conformance tests, not idea but better than skipping all conformance tests as we do now. Portals render other components, which handle the as prop anyway.
• Look for the user's className in the document.body if rendering a portal.

[codecov-io]

Codecov Report

Merging #2146 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2146   +/-   ##
=======================================
  Coverage   99.76%   99.76%           
=======================================
  Files         150      150           
  Lines        2601     2601           
=======================================
  Hits         2595     2595           
  Misses          6        6

Impacted Files	Coverage Δ
src/modules/Popup/Popup.js	100% <ø> (ø)	⬆️

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[levithomason]

Released in semantic-ui-react@0.75.0