Small usability improvements for the unit tests

[brian-pane]

• If a test fails due to a missing input file, print the pathname.
• Document the needed thirdparty submodules in README.md.
• In the documentation comment on rsa::SigningKey, put the example file in a temp directory that gets cleaned up automatically upon test completion, so that running the tests will no longer leave a key.der file in the local ./graviola directory.

[codspeed-hq]

CodSpeed Performance Report

Merging this PR will not alter performance

_{Comparing brian-pane:testing (05e7fba) with main (fca0ed5)}

Summary

✅ 143 untouched benchmarks

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.18%. Comparing base (fca0ed5) to head (05e7fba).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #130   +/-   ##
=======================================
  Coverage   99.18%   99.18%           
=======================================
  Files         170      170           
  Lines       39158    39159    +1     
=======================================
+ Hits        38838    38839    +1     
  Misses        320      320           

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