Add unit tests for mounts of delete users

[nickvergessen]

Downstream of 27488

[MorrisJobke]

The acceptance test selenium container has thrown a weird error. I restarted - let's see if this was due to a little hiccup in the network.

[codecov]

Codecov Report

Merging #4473 into master will increase coverage by <.01%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master    #4473      +/-   ##
============================================
+ Coverage     54.11%   54.11%   +<.01%     
  Complexity    21679    21679              
============================================
  Files          1331     1331              
  Lines         83053    83053              
  Branches       1310     1310              
============================================
+ Hits          44941    44945       +4     
+ Misses        38112    38108       -4

Impacted Files	Coverage Δ	Complexity Δ
lib/private/User/Manager.php	83.81% <0%> (-0.58%)	68% <0%> (ø)
lib/private/Files/Config/UserMountCache.php	89.13% <0%> (+0.72%)	40% <0%> (ø)	⬇️
lib/private/Files/Cache/Propagator.php	96.2% <0%> (+1.26%)	16% <0%> (ø)	⬇️
apps/files_trashbin/lib/Expiration.php	91.93% <0%> (+1.61%)	29% <0%> (ø)	⬇️
apps/comments/lib/EventHandler.php	87.5% <0%> (+8.33%)	7% <0%> (ø)	⬇️