tests: fix flaky fetcher, catchpoint, expect tests

[algorandskiy]

Summary

1. Fix a race condition in TestRequestBlockBytesErrors as seen here. Caused by a specific assert when running block fetching on pre-cancelled context. In this case Request races with async Respond so both response channel and context.Done are ready. Fixed by adding context cancellation pre-check in the function prologue.
2. Fixed GLOBAL_TEST_ALGO_DIR clearing in expect tests causing error like this.
3. Fixed catchpoint content mismatch in TestCatchpointAfterTxns by forcing trackers to flush before creating catchpoints.
4. Fixed a race in double ledger content verification in TestExactAccountChunk as seen in this master branch build failure

Test Plan

Flaky test fixes

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 47.76%. Comparing base (c80fa40) to head (68ebf79).
⚠️ Report is 15 commits behind head on master.
✅ All tests successful. No failed tests found.

❗ There is a different number of reports uploaded between BASE (c80fa40) and HEAD (68ebf79). Click for more details.

HEAD has 5 uploads less than BASE

Flag	BASE (c80fa40)	HEAD (68ebf79)
full_coverage	5	0

Additional details and impacted files

@@             Coverage Diff             @@
##           master    #6546       +/-   ##
===========================================
- Coverage   61.89%   47.76%   -14.14%     
===========================================
  Files         484      645      +161     
  Lines       67571    87846    +20275     
===========================================
+ Hits        41822    41957      +135     
- Misses      22184    43131    +20947     
+ Partials     3565     2758      -807     

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