stack controller: watch for delete events

[EronWright]

Proposed changes

Ensures that progress is made if the Workspace object is deleted concurrently with the Stack object, as may occur in foreground deletion. Otherwise the operator stays idle when it should reconcile.

A partial solution is to have the Stack controller watch for Workspace 'delete' events, since the controller is otherwise stuck waiting for the workspace to become ready.

Or, the stack controller may have seen the workspace as ready, and proceeded to create an update for a non-existent workspace, which will not progress because the update controller waits for the workspace.

Also: fix a flaky test.

Related issues (optional)

Closes #753

[codecov]

Codecov Report

Attention: Patch coverage is 40.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 50.75%. Comparing base (bbbad20) to head (bb63989).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
...tor/internal/controller/pulumi/stack_controller.go	25.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #756      +/-   ##
==========================================
+ Coverage   50.73%   50.75%   +0.02%     
==========================================
  Files          30       30              
  Lines        4173     4175       +2     
==========================================
+ Hits         2117     2119       +2     
  Misses       1872     1872              
  Partials      184      184              

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