Tweak runTests() output format

[alandipert]

Currently runTests() returns a structure with a member that varies in type between a result value, NA, or an error object.

This PR normalizes the format of the return value of runTests() at the cost of some redundancy. The goal is to simplify consumption by downstream tools interested in test outcomes.

Dev TODO

• Add test(s)
• Improve documentation
• Add to NEWS.md

[alandipert]

CI is currently failing with the following R CMD check NOTE:

* checking installed package size ... NOTE
  installed size is 11.1Mb
  sub-directories of 1Mb or more:
    R     1.6Mb
    www   8.3Mb

[schloerke]

That is a standard/consistent NOTE for shiny