Fix strange gui case name appearing for each run of EnsembleSmoother

[DanSava]

Issue
Resolves #5141

Approach
Replace the gui name with current selected case name with a _prior suffix. So if a user runs EnsembleSmoother with the current case set for the default case. after the smoother runs two new cases are created:

1. default_prior
2. default_smoother_update

Pre review checklist

• Added appropriate release note label
• PR title captures the intent of the changes, and is fitting for release notes.
• Commit history is consistent and clean, in line with the contribution guidelines.
• Updated documentation
• Ensured new behaviour is tested

Adding labels helps the maintainers when writing release notes. This is the list of release note labels.

[codecov-commenter]

Codecov Report

Merging #5223 (a8cab2f) into main (b5de566) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #5223   +/-   ##
=======================================
  Coverage   72.34%   72.34%           
=======================================
  Files         394      394           
  Lines       27439    27439           
  Branches     2007     2007           
=======================================
  Hits        19851    19851           
  Misses       7024     7024           
  Partials      564      564           

Impacted Files	Coverage Δ
src/ert/gui/simulation/ensemble_smoother_panel.py	95.00% <ø> (ø)

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more

[andreas-el]

This seems like a fair solution. Undoubtedly better than what was already there. 👍