[Tests-Only] Fix unit tests related to core issue 37358 and core PR 37103

[phil-davis]

Part of issue owncloud/core#37358

The signature of core lib/private/User/Manager.php get() changed to have a new 2nd parameter.
See core PR owncloud/core#37103

Adjust unit tests so that the mocking of userManager matches the way that phpunit sees the calls. Even though the new parameter is optional (has a default value), phpunit sees the call happening with the parameter having been set to its default value false

[codecov]

Codecov Report

Merging #337 into master will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #337   +/-   ##
=========================================
  Coverage     83.34%   83.34%           
  Complexity      316      316           
=========================================
  Files            22       22           
  Lines          1015     1015           
=========================================
  Hits            846      846           
  Misses          169      169           

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