Keep testing configuration alive after losing focus

[ericsnowcurrently]

(for #4288)

Note that I changed a few places to use IApplicationShell instead of vscode.window. This isn't strictly related to the PR (I thought it was at first), but it's not a big addition so I'd rather just leave it. If there are any objections then I don't mind dropping that part (i.e. the "Use IApplicationShell instead of vscode.window." commit below).

• Pull request represents a single change (i.e. not fixing disparate/unrelated things in a single PR)
• Title summarizes what is changing
• Has a news entry file (remember to thank yourself!)
• Has sufficient logging.
• [ ] Has telemetry for enhancements.
• [ ] Unit tests & system/integration tests are added/updated
• Test plan is updated as appropriate
• [ ] package-lock.json has been regenerated by running npm install (if dependencies have changed)

[codecov]

Codecov Report

Merging #4316 into master will increase coverage by 23%.
The diff coverage is 25%.

@@           Coverage Diff            @@
##           master   #4316     +/-   ##
========================================
+ Coverage      57%     79%    +23%     
========================================
  Files         348     422     +74     
  Lines       14566   19363   +4797     
  Branches     2252    3112    +860     
========================================
+ Hits         8263   15257   +6994     
+ Misses       5774    4103   -1671     
+ Partials      529       3    -526

Flag	Coverage Δ
#Linux	68% <25%> (?)
#Windows	69% <25%> (?)
#macOS	68% <25%> (?)