Added action buttons to top of Test Explorer

[karrtikr]

For #4275

• 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 #4327 into master will increase coverage by 22%.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           master   #4327     +/-   ##
========================================
+ Coverage      56%     78%    +22%     
========================================
  Files         361     436     +75     
  Lines       15272   20244   +4972     
  Branches     2373    3246    +873     
========================================
+ Hits         8494   15612   +7118     
+ Misses       6234    4629   -1605     
+ Partials      544       3    -541

Flag	Coverage Δ
#Linux	67% <ø> (?)
#Windows	67% <ø> (?)
#macOS	66% <ø> (?)

[DonJayamanne]

Please use svg files instead of png files.
Use files from VSC itself, here are the icons:

• Play/start (dark & light) = https://github.com/Microsoft/vscode/blob/master/src/vs/workbench/contrib/debug/browser/media/start.svg

• Debug Dark - https://github.com/Microsoft/vscode/blob/master/extensions/npm/resources/dark/debug.svg

• Debug light - https://github.com/Microsoft/vscode/blob/master/extensions/npm/resources/light/debug.svg

• Discover (use refresh icon) - https://github.com/Microsoft/vscode/blob/master/extensions/npm/resources/light/refresh.svg
  & https://github.com/Microsoft/vscode/blob/master/extensions/npm/resources/dark/refresh.svg

• Play Failed (we need to generate a red icon based off the green SVG). Should be straightforward enough.

• Problems (failed)

[karrtikr]

• Problems (failed)

@DonJayamanne Can you elaborate? The issue doesn't state this. Should i have a button for that?

[DonJayamanne]

• Problems (failed)

Please ignore that comment.

[DonJayamanne]

Please use the same names and images from vscode:

• use start.svg instead of play.svg
• use debug.svg instead of debugger.svg
• use start-failed.svg instead of playFailed.svg

Please ensure you test the icons in both light and dark themes of VS code.