[#64557522] Coverage only for unit/spec tests


with reference to 
https://ruby.awsblog.com/post/Tx1GAYRAS8PX4CC/Using-SimpleCov-with-Multiple-Test-Suites (and advice from @annashipman and @alext )

* guard the coverage run with an ENV var check
* set the ENV var in the rake target for the unit/spec tests