[Tree][NestedSet] Fix inserting multiple roots in one flush

[sgehrig]

This PR tries to solve the issue described in #2582. Currently it only provides a test case that reproduces the issue and the observations made in #2582 (comment) (persisting roots first) and in #2582 (comment) (doing two flushes).

\Gedmo\Tests\Tree\Issue\Issue2582Test::testInsertTwoRootsInOneFlush fails as expected right now because it is a direct replication of the issue. \Gedmo\Tests\Tree\Issue\Issue2582Test::testInsertTwoRootsInOneFlushRootsFirst and \Gedmo\Tests\Tree\Issue\Issue2582Test::testInsertTwoRootsInTwoFlushes both succeed.

[sgehrig]

The change in \Gedmo\Tree\Strategy\ORM\Nested fixes the issue and keeps all tests green. I am not sure however if this is in any shape or form correct.

When debugging the failing test case I noticed that the \Gedmo\Tree\Strategy\ORM\Nested::$treeEdges property is not updated when \Gedmo\Tree\Strategy\ORM\Nested::updateNode handles the second non-root node.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.44%. Comparing base (211b6fe) to head (2156abe).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2932      +/-   ##
==========================================
- Coverage   78.53%   78.44%   -0.09%     
==========================================
  Files         168      168              
  Lines        8790     8809      +19     
==========================================
+ Hits         6903     6910       +7     
- Misses       1887     1899      +12     

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[sgehrig]

@phansys Is there anything I need to do to get this one merged? Thanks a lot for your help.