Adhoc concurrent test for program cache

[LucasSte]

Problem

There are no concurrent tests for the program cache. We cover most lines in our testing, but the line that stops a thread on a condition variable has no testing coverage.

I noticed that this is the only place where there is an explicit wait/notify condition. In all other places in the program cache, we use mutexes and RWLocks, which are already covered in basic unit tests. Adding concurrent tests for them would only mean covering their blocking functionality.

Summary of Changes

I included an impromptu test to cover the cookie line in replenish_program_cache. The test is adhoc and is only a stop gap until I configure the repository to receive shuttle, which provides with mechanisms to manipulate thread scheduling and ensure correct multithreaded tests.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.8%. Comparing base (e86ab5a) to head (c39034a).
Report is 65 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff            @@
##           master    #1471    +/-   ##
========================================
  Coverage    82.8%    82.8%            
========================================
  Files         872      872            
  Lines      369310   369435   +125     
========================================
+ Hits       305846   306010   +164     
+ Misses      63464    63425    -39     

[LucasSte]

The codecov report shows the cookie line is now covered: https://app.codecov.io/gh/anza-xyz/agave/commit/c39034a846a71d58d25ac9751cf641b968b0187e/blob/svm/src/transaction_processor.rs#L454

[Lichtso]

This is probably the fourth or fifth time we implement a program load to deployment for tests method. Should be unified, but that can happen in a separate PR.

[LucasSte]

I guess the issue is that all loading functions are under a #[cfg(test)] flag, which makes them unavailable for import.