clear collection titles before updating

[elfkuzco]

Rationale

This PR clears the collection titles before creating the new ones to be used for an update. By clearing the old collection titles in memory, we avoid creating duplicate collection titles when the original collection title is sent in the update reqeust. This is because SQLAlchemy list of collection titles isn't cleared with just session.delete.
As a consequence, having duplicate collection titles leads to creating duplicate book locations which violates location unique constraint.

Changes

• clear collection titles before adding new ones
• update tests to model real-world scenario such that title to update has book, collection title, etc.
• remove redundant second test since updated test covers everything

This fixes #277

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.37%. Comparing base (6408a8e) to head (7f495bd).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #280      +/-   ##
==========================================
+ Coverage   81.28%   81.37%   +0.09%     
==========================================
  Files          55       55              
  Lines        2362     2363       +1     
  Branches      236      236              
==========================================
+ Hits         1920     1923       +3     
+ Misses        381      380       -1     
+ Partials       61       60       -1     

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[elfkuzco]

@benoit74 , I am wondering if this is not a good time to add a unique constraint to the collectiontitle table too. The keys for the constraint will be title_id, collection_id, and path. WDYT?