Async signing test util follow ups

[alecchendev]

Some followups on #3115.

• Remove unused unavailable signers in TestKeysInterface
• Remove unnecessary display implementation for SignerOp
• Move set of test disabled signer ops to EnforcementState
• Add method to enable/disable all signer ops at once

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 17.07317% with 34 lines in your changes missing coverage. Please review.

Project coverage is 89.74%. Comparing base (3ccf064) to head (1ce5bbd).
Report is 2 commits behind head on main.

Files	Patch %	Lines
lightning/src/util/test_channel_signer.rs	17.24%	20 Missing and 4 partials ⚠️
lightning/src/ln/functional_test_utils.rs	0.00%	10 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3151      +/-   ##
==========================================
- Coverage   89.81%   89.74%   -0.07%     
==========================================
  Files         121      121              
  Lines       99314    99331      +17     
  Branches    99314    99331      +17     
==========================================
- Hits        89195    89141      -54     
- Misses       7514     7532      +18     
- Partials     2605     2658      +53     

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[TheBlueMatt]

LGTM, its only test code so not bothering to wait for another review.