Add unit tests for mutex classes

[WarLikeLaux]

Q	A
Is bugfix?	❌
New feature?	❌
Tests added?	✔️
Breaks BC?	❌
Fixed issues

What does this PR do?

Adds unit tests for Mutex, DbMutex, and OracleMutex.

Coverage

File	Tests	Lines	MSI
framework/mutex/Mutex.php	7	15/17 (88%)	86% (20/23)
framework/mutex/DbMutex.php	5	2/2 (100%)	0% (0/1)
framework/mutex/OracleMutex.php	8	3/28 (11%)	61% (8/13)

Changelog not required, because no source files changed.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.63%. Comparing base (068c5a4) to head (0e762fd).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #20786      +/-   ##
============================================
+ Coverage     78.62%   78.63%   +0.01%     
  Complexity    11566    11566              
============================================
  Files           376      376              
  Lines         30748    30748              
============================================
+ Hits          24176    24180       +4     
+ Misses         6572     6568       -4     

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