fix: modify test_simple_signer to fix intermittent failure

[hstove]

#4280 modified a test in libsigner to test the /status endpoint. Because I didn't realize that the test signer thread was configured to exit after 5 events, I designed the test wrongly. Tests would pass locally and sometimes on CI, but I realized later that the tests would sometimes fail in CI because a thread would receive an event before it was shutdown.

This PR:

• Removes the /status endpoint testing in the original test
• Adds a new test that's specifically for the status endpoint

[codecov]

Codecov Report

Attention: 4 lines in your changes are missing coverage. Please review.

Comparison is base (bdb2e3c) 83.38% compared to head (119caa6) 83.27%.

Files	Patch %	Lines
libsigner/src/tests/mod.rs	90.69%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             next    #4306      +/-   ##
==========================================
- Coverage   83.38%   83.27%   -0.12%     
==========================================
  Files         434      434              
  Lines      308770   308799      +29     
==========================================
- Hits       257482   257142     -340     
- Misses      51288    51657     +369     

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